B&L Photomicrographic Apparatus VI: AX-1

The final development, before the microscope camera went from still-chemical photography to motion/still digital photography is equal parts groundbreaking and re-purposing. Like the System II the new version maintained the same basic layout and universality, it was made compatible with all the camera backs of the System II. It retained the focusing knob (to date all those known are marked at 3x, 5x, 7.5x, and 10x) but departed from the consumer camera style shutter speed selector and remote shutter release; the new camera was automated.

Since the earliest days of photomicrography it was just assumed the the microscopist would be the darkroom technician as well. Those seeking to record images of their work were expected to practice and understand the skills needed not only for obtaining a clear, useful image at the eyepiece, but at the imaging surface of their camera, and final media as well. Books from the golden age are filled with detailed information on how to properly compose and sequentially expose test images. Entire chapters in books on general microscopy are devoted to the proper taking of photomicrographs. In detailed texts on photomicrography extensive chapters describe the best way to salvage (and in future avoid) over or under-exposed films. The AX-1 changed all that.


For a generation that grew up (or like the author, grew complacent) with the automatic exposure and aperture cameras of the “Kodak Moment” era, the AX-1 no doubt seems unduly complex. A two variable analog meter, no less than six buttons-two thirds of which light, two additional indicator lights, four tumbler set values, all run off of a grounded AC plug in a package the size of a lunch box and weight of a bag of sugar is understandably daunting. The space-hogging controller is only half the unit and like it’s counterpart is entirely useless without the automatic shutter assembly camera. As previously mentioned it is superficially like the System II but in place of a release bears a nine pin cable and connector that conducts light data from its integrated sensor to the control box and carries the exposure signals back. A small electromagnet actuates the simple shutter, and the photo-sensor is positioned to one side of a right angle partially silvered reflector.

To allow for the proper calculation of required exposure one must set a few parameters on the controller before beginning. Film speed is dialed in using A.S.A. units and it’s worth noting that the controller uses the 1960 revision (from what I can tell in use, I’ve not been able to locate anything in the documentation). One must also select the desired reciprocity, which is best left set to “OFF” for most uses. Setting the level of magnification to the nearest value marked on the shutter informs the photo-sensor of the general range of light intensity and is vital for achieving accurate exposure times and light levels. One may also set the desired exposure to a fixed value darker or lighter than the calculated exposure which may be useful for those who want to avoid pushing or pulling during processing.

Once properly set up the user is ready to set the focus of the shutter assembly, load film, power up and begin taking pictures at the push of a button! Imagine, no need to shoot and develop costly trial exposures or to calculate required shutter speeds/exposure times by hand with based on light meter readings. The device can serve as a light meter only and exposure may be done manually at the press of a button. The controller can be put on a separate table and the only source of vibration limited to the movement of the shutter itself. That’s probably the chief point in the AX-1’s favor these days, but it may still be worth while for those more willing to give it the space than to learn about shutter speeds.


B&L Photomicrographic Apparatus V: System II

A Re-design

The System II represents a fundamental change in B&L integrated photomicrographic cameras. Sleeker than the early DynaZoom camera but far more substantial than the later versions, the System II is a step back to the design principles that required a focusing system. It was necessary for the System II to feature a focusing lens system because the unit was intended to be a new universal device for all (camera ready) professional B&L lines, including the earlier DynaZoom/DynOptic lines. Despite maintaining the compatibility of the system with earlier stands, once more the camera backs required are not compatible with previous versions (although they do come in all previous formats).

New Features

The opticians behind the System II seemingly asked themselves ‘how can we make this the last camera a microscopist will need?’ and set about answering it. The systems chief major departure is the coupler, specifically its diameter. At 7/8ths it’s a full quarter inch larger than the previous model DynaZoom camera, and a return to the large size used with focusing DynaZoom cameras. Care was also taken in the design of the coupler, it was made to be removed and a number of versions of the coupler were made, some making allowance for the use of a manual exposure meter.

Once more the camera back attachment point was redesigned. The new male dovetail fitting was made a bit smaller than those used previously and unlike previous models did not feature an alignment screw that required the female dovetail camera back be fixed in relation to the camera. Additionally, where earlier camera backs used two thumb screws the new versions used only one; making it a simple thing to orient the photosensitive surface however one wished without being forced to suffer an inconvenient shutter release or winder position.

Couplers and Camera Backs

Three couplers are known of: the silver Bal-Coated male dovetail, the black lens-less male dovetail, and the eyepiece mount. The silver dovetail makes good use of a Bal-Coated lens to eliminate errant reflection which can prove frustrating with uncoated optics and both it and the black dovetail carry a small slot in the dovetail that mates with the camera. This slot may be turned to align with a similar slot in the System II or turned so as to exclude all light. The purpose of the slot is to permit the insertion of the B&L Cat. # 421240 exposure meters sensor.


The versions are discrete rather than explicit, which is to say there is not any particular model number to distinguish them that is official but they are readily identifiable. One sort is focusable from below 3x to over 10x. On these models only four magnifications are marked: 3x, 5x, 7.5x, and 10x. Two versions of this sort are known, one having an all black shutter speed selector and the other having a silver ring. An additional version, and perhaps more desirable (if less common) one has the same range but is marked in one unit increments from 0-25x with every fifth mark being numbered.


Apart from the previously mentioned camera backs a number of accessories were produced. Special viewfinder adapter plates and eyepieces were available for the various camera backs. It should be noted that these eyepieces were not the film area viewfinder eyepieces that would show the field that would be covered by various popular film sizes. Extension tubes were also available that would extend the distance between the final lens element of the shutter assembly and the camera back. Tubes were made in 2.5x and 5x lengths were produced and could be used in various combinations where special needs required.


∗It should be noted that the connection to the shutter in all cases is a male dovetail and the designation here refers to the connection to the microscope photo-port.

†I’ve seen one in use during the course of a museum visit although I’d be hard pressed to say if it were an actually B&L item or the product of a machinist and a tight budget.

B&L Photomicrographic Apparatus III: Integrated Cameras

Microscope Stand Design

Around the halfway point of the last century (1940 or so) microscope design began to change drastically. Modern manufacturing and engineering, together with the proliferation of compact, bright, electric lamps contributed to an opticians ability to design microscopes which provided a new option for the photomicrographer. Contemporarily advertised as tri-nocoular (but more properly referred to as photomicrographic) microscopes began to become available with specialized camera ports that did not require interchanging a binocular body for a monocular body, or the occupation of a standard eyepiece tube by a cumbersome attachment camera.

At the time this was a considerable advance and as photographic process became more accessible and streamlined so too did the process of making ones microscope ready to capture an image. In many cases a microscope could be used visually as normal and simultaneously outfitted with appropriate apparatus capture images on a variety of media. Ones microscope could stand ready for either photomicrography or visual use and a great many sights that were only previously fleeting could be captured.

Regrettably, while some mechanical aspects such as the Royal Microscopical Society (RMS) thread, or 23.2mm eyepiece have become standard, the same can not be said of photomicrographic apparatus. For obvious reasons apparatus from one manufacture may not be compatible with another. Infuriatingly, apparatus from a given manufacturer may not be immediately compatible with a different line or generation of microscopes.

The Systems

It can be impossibly difficult to find information on the integrated camera systems of Bausch & Lomb, so forgive any inaccuracies and permit the establishment of two broad categories; the DynOptic/DynaZoom (hereafter DynaZoom camera) and the Integrated Camera System II (hereafter System II). Each was available with a range of formats and in a variety of styles with various interchangeable or permanently fixed accessories.

The DynaZoom camera was created specifically for the so named line of microscopes and was available throughout the life of that line. When the black finish stands were replaced by the slate finish models the cameras followed suit but maintained the established standards and are interchangeable with the earlier versions and vice versa.

The System II was in simultaneous production with the DynaZoom but was created so as to be compatible with a wide range of microscope lines and to offer improved options and forward compatibility. While still compatible with both the black and slate DynaZoom microscopes it could also be fitted to StereoZoom, and BalPlan microscopes as well as Bausch & Lomb Bench Metallographs, and inverted microscopes. System II also saw the introduction of automatic shutter control with electronic light metering.

One may immediately identify the camera system because in the DynaZoom cameras of every sort there is no independent magnification (focus) control. System II cameras of every sort feature a vertically oriented magnification (focus) control which may be marked from 0 too 25 or from 3x too 10x.

Sorry for the long silence, computer failure and a new job have eaten up a great deal of my time lately. I promise the next post wont be so far away. -K

B&L Photomicrographic Apparatus II: Attachment Camera

The Attachment Camera

The attachment camera as designed by Bausch & Lomb remained primarily the same over its production life. Changes to the apparatus itself were minor and all components have proven interchangeable if one is faced with assembling a complete working device from multiple units purchased separately for parts. The shutter features the following speed settings: 1/10, 1/25. 1/50, 1/100, 1/200, B, and T, and uses a standard cable release. There is no prism release as may be found on attachment cameras from other manufactures. This is a bit of a trade off, as without the prism release one can not direct 100% of available light to the camera. Fortunately, absence of a prism does eliminate its motion as a source of vibration during exposure. In place of a swing out prism a preferential beam splitter is fitted which directs 85-90% of available light to the camera and from 10-15% to the viewer. Aside from the camera itself (see below) the part most likely to be missing is the ocular sleeve which is a metal collar that slips over the tube of the microscope prior to the fitting of an ocular. The sleeve holds the shutter assembly in alignment with the optical axis of the microscope and holds the beam splitter at the exit pupil of the ocular.

This design requires a vertically oriented microscope tube. If used with a classic style B&L Dynoptic binocular microscope one will need to obtain the interchangeable monocular body. When attached the viewfinder projects horizontally towards a seated microscopist. There are two varieties of viewfinder both projecting significantly from the axis of the microscope so that the microscopist need not lean in uncomfortably when using the camera. Each sort makes use of a ground glass screen which the older version views through a large fixed focus condenser. The newer variety uses a spiral focus condenser that improves the clarity which one obtains to a certain extent, but does require that the user lean in towards the lens for proper viewing. Focus is entirely via the controls of the microscope.


The camera bellows and bodies that are available for the unit run the gamut. Most common of the camera attachments is the 35mm camera body. The camera itself is often quite diverse and might be any of a half dozen different models. As of this writing the author has found the following 35mm camera bodies: Argus (stripped down, unknown model), Kodak Pony (without view finder), Kodak ColorSnap (complete with permanent lens mount). There are two lengths of bellows tube for the 35mm camera, one providing a 5x enlargement, the other 10x. Each tube is equipped with a dark slide.

Less common is the 2¼ x 3¼ cut film holder which is not equipped with a dark slide. The fixed enlargement factor metal bellows cone is felt lined internally and is confirmed as compatible with Kodak film holders and plates. There may or may not have been a ground glass focusing screen but considering the standard size of the film holder it is simple enough to obtain one from a third party source. Thankfully, a ground grass screen is not necessary as the film plane is parfocal with the fixed focus ground glass viewer. Film is only infrequently available from a limited number of suppliers but may be desirable if one has access to a dark room and a willingness to cut down more commonly available film stock. It is not compatible with the common 120 roll film Graflex camera backs without modification to the camera back or a homemade light seal.

The 2¼ x 3¼ Polaroid instant film pack camera backs may be found and makes use of the 10 exposure peel apart film now produced by Fuji. Although the images obtained are very nice, and provide an easy introduction to medium format photomicrography, the difficulty of pulling film from the camera (so that the chemistry is applied to the film) will require verification of alignment between exposures. The commonly available backs compatible with Graflex cameras are not compatible with the above described bellows. An additional model permanent camera is fitted to a plastic fixed enlargement factor bellows cone and is not outfitted with a dark slide. Unless one has a Polaroid back that is known to be interchangeable with a Kodak film holder it is advisable to obtain the permanent Polaroid version if one is set upon using that format.

The 4 x 5 camera bellows is infrequently seen but provides the best option for those interested in medium or large format photomicrography. A scaled up version of the 2¼ x 3¼ camera, it is metal and felt lined and like it’s smaller counterpart, also does not have a dark slide. The cone is compatible with 4 x 5 camera backs, of all sorts including cut film, film pack, and plate holders. Off the shelf 120 roll film backs from Graflex are compatible and the ease of using such a system can be an economical way for the beginner to get involved with large and medium format chemical photomicrography. Depending on the winder it may be necessary to remove the metal spring that holds cut film plates in place. Ground glass focusing screens are simple to find or make with a bit of glass and a dab or carborundum grit, but the original screen features a thick rubber frame.


No doubt everyone can picture the attachment camera in use on a classic monocular microscope. In the interest of displaying something one might not have considered, below is an image of the attachment camera (with Polaroid back and spiral focus viewfinder) in service on a Phase Contrast DynaZoom as well as two detail images of the difficult to find ocular sleeve.



∗Images in reference works term the apparatus the “Model N” attachment camera but I have not been able to locate the appropriate B&L catalog or manual to confirm the proper name.

†The camera back is slightly too wide and thick. A few minutes with a hobby grinder or similar tool is all that is needed to permanently adapt a roll film back. If one is unwilling to dedicate a camera back, a thick felt gasket may be glued to the back and cut so that it projects slightly into the bellows cone to prevent it from being jostled while in use.

‡The hardest part is cutting the glass to the right size and it isn’t even that hard if one has ever cut glass.

On Photomicrographic Apparatus

Modern Variety

There is a staggering amount of digital photomicrographic equipment. Modern student or college level microscopes outfitted for photomicrography have moved away from the previous standard of the tri-nocular head with detachable camera, and are now frequently found with dedicated integral digital cameras. The downside of such cameras aside from the added initial expense (which may double the cost of a comparable binocular microscope) is the inability to upgrade to a superior camera as advances are made or funds become available, and the software requirements. The upside is the possession of a tailored optical system that (ostensibly) has been designed with a knowledge of the microscopes optical system.

The contemporary alternative is the digital eyepiece camera or the eyepiece relay adapter. Despite the versatility of such things their occupation of an eyepiece may greatly complicate the normal use of the microscope. Such secondary optics might just as easily result in inferior image quality depending on the optical system of the microscope. Various artifacts and aberrations may be unavoidable and not immediately apparent.

Classic Variety

Putting aside the issue of format for a moment, classic photomicrographic equipment falls into the same two broad categories as modern digital photomicrographic gear. The traditional attachment camera was designed for use with monocular microscopes and many binocular microscopes were available with an interchangeable monocular body to be used with the photomicrographic outfit. Such apparatus was generally designed so as to use an ocular which could be varied depending on the objectives employed or magnification desired. Unlike modern equivalents most used a beam splitter to provide a viewing port for focusing, those which did not made use of a ground glass screen which could be interchanged with the film or plate holder.

The second category relied upon specialized microscope viewing heads which were designed with a beam splitter which would send a portion of the light into a photomicrographic camera system. Nearly all lines of microscopes by the major houses were available with photomicrographic heads of different sorts encompassing the solely photomicrographic (without provision for visual use) to the monocular and binocular photomicrographic (trinocular). Such microscopes relied on the use of compatible photomicrographic systems generally provided by the manufacturer.


An off-the-shelf microscope equipped for digital photomicrography will undoubtedly function but one risks being tied to an overly simplistic microscope that does not meet the requirements of the microscopist as they grow in their ability. The same might be said of a modern or classic microscope outfitted with a digital camera and an eyepiece adapter, with the added difficulty of uncertainty of the suitability of the system. For modern photomicrographic equipment then the chief hazards is inferior equipment and the easy of excessive digital image modification.

Classic photomicrographic gear is only very infrequently available as a complete system in good working order. Obtaining results that do it justice is often heavily reliant on the ability of the user to locate and properly employ gear which was originally designed for use with a given microscope line. Such efforts are often complicated by the lack of relevant documentation or informed sellers§ who greatly increase the difficulty of locating equipment by selling it under some other designation. Recognizing compatible equipment on sight is inherently difficult. Attachment cameras using chemical photographic formats provide resolution that well exceeds consumer grade digital cameras, but one suffers for the time and expense of development and processing of significant numbers of test photomicrographs while getting the equipment in working order.


One can obtain a perfectly suitable modern microscope and outfit it for digital photomicrography at a reasonable cost, assuming of course the images are not required to be used for serious work. A student microscope designed for entry level photomicrography might easily run $500.00 US and one suitable for college level work could exceed $1000.00 without batting an eye. Regrettably, such microscopes are generally inferior to used professional microscopes of similar cost, but for assurance of capacity one could not be faulted for going that route.

A used professional grade trinocular microscope (such as the B&L DynaZoom, DynOptic, BalPlan, AO Spencer 10, Microstar, or 2/4) from any of the major houses may generally be found for far less than all but the cheapest modern student microscopes. Many microscopists have no interest in chemical photomicrography, however, what is seldom considered is that such microscopes as mentioned above existed at a time when the C-mount video camera was in wide use. After obtaining a trinocular microscope one should endeavor to find the c-mount photo-tube sold by the original manufacturer. This author is personally aware that such tubes exist for Bausch & Lomb and AO Spencer, an is told it is true of Olympus, Ziess, and Nikon as well.

One may just as well go the above route and in place of a difficult to find c-mount tube purchase a widely available Polaroid instant camera system designed for the microscope. Film remains available for the more common formats and one has all the benefits of chemical photography without the need of expensive darkrooms or tedious developing processes.


∗As distinct from antique photomicrographic equipment which should be taken to include the bellows outfitted horizontal and vertical cameras.

†Alternatively the microscope head might be equipped with a control which would direct 100% of available light to either the ocular(s) or the photomicrographic outfit.

‡By this is meant the difficulty of using a system that can not be tested before hand—most equipment must be purchased on the internet—and the difficulty of recognizing unforgivable visual artifacts and aberrations as a beginner.

§Most of the authors photomicrographic equipment was obtained from dealers who incorrectly identified it as projection or aerial photography gear.

Projection Microscopy III

The Set Up

The 1 Inch is old enough to be corrected for a 10 inch tube length so when setting up for micro-projection one must be sure of two things; first, that if an eyepiece is used that the draw tube is able to provide for that distance, and second, that the projection distance is measured from a point 10 inches beyond the rear lens of the objective if an eyepiece is not used. For simplicity a dedicated projection microscope will be used for testing. It is a Bausch & Lomb dating to 1956 and was intended for use with its own projection objectives (distinct for the orientation of the B&L logo on the objective barrel). The intensely bright lamp integral to the stand will have no difficulty providing adequate light.

According to published information from the manufacturer contemporary to The 1 Inch, one knows that a minimum projection distance of 15 feet is recommended for the most highly corrected image. A projection surface (in this instance a handy, blank, artists canvas) was positioned 16 feet from the stage of the projection microscope. With the head of the microscope properly aligned a projection distance of just over 15 feet was obtained.


It’s would appear that obtaining a digital photograph of a projected image is not a job to be entered into lightly or in haste. Apologies then, but please understand that in person the image produced by the objective is in a word: astounding.

Stained section of human skin

Stained section of human skin.

Palp of wolf spider mounted whole with pressure.

Palp of wolf spider mounted whole with pressure.

The low light conditions and poor handling of the same by the author and his 14 year old digital camera do such little justice I considered not including the images at all. For several hours (until other responsibilities intruded) various specimens demanded projection and rapt attention. During this time there was no noticeable heating of the objective despite the intense heat of the lamp. In fact having previously made use of the objectives provided with the stand I can say that The 1 Inch remains cooler than those of much more modern construction.

Color correction is as better than that provided by any objective alone that I have seen. In spite of the long projection distance flatness of field was observed to be quite profound. Using a wall as the projection surface, images spanning 5 feet were observed to be in nearly uniform focus across their entire diameter.

I will have to read up on low-light photography in the hopes of sharing more accurate portrayals of the abilities of The 1 Inch. -K

Projection Microscopy II

An antique B&L projection objective of 1 Inch focus

An antique B&L projection objective of 1 Inch focus

An Old Objective

The above is a Bausch & Lomb objective (hereafter dubbed The 1 Inch) of indefinite age. It is shown with its canister and marked as follows: “Bausch & Lomb Optical Co.” in the old style gothic lettering, “ROCHESTER, N.Y.” in uniform san-serif block upper-case capitals, “Projection” in large first letter upper case serif capitals, and finally designated “1 In.” in the same style beneath that. One acquainted with the Bausch & Lomb line will immediately recognize it as dating to before the 1930 catalogue, when objectives were listed by power and equivalent focus in millimeters. That the focus is marked in inches one will quickly note places it at least in the era of the 1899 “Catalogue A” except that at this time the objectives were marked also with the tube length. It is likely then that the objective is older than 1899, and that being the case, further likely that the tube length is the traditional long form (10 inches or 250mm) rather than the short (160mm) to which one will be accustomed. Thankfully, the objective is to be used for projection and dates to a period when oculars were not required (though could be employed) for projection or photomicrography.

Taken as it stands with only the marked information known one can take 10x for the magnifying power of the objective. It was simple in the days of the ten inch tube length to calculate the power of an objective from its imperial focus integer be that whole or fractional; a ten inch tube length is divided into marked focus of the objective providing a designation of magnifying power. To anyone accustomed to shorter tube length objectives The 1 Inch will seem of unusual size for a low power. A certain amount of this attributable to the longer tube length for which it was corrected, though most is due to its mechanical construction as dictated by its use.

Its Build

The 1 Inch is housed in a traditional brass RMS threaded body, but one will note immediately the oddness of its front element. Robustly constructed of black metal without a finish, the component is less an optical element and more a shield. It may be unscrewed to reveal a recessed front lens which is nearly twice the size of the aperture in the shield. Further investigation shows that the front and rear lenses of the objective are separated by a considerable distance. The combination nearest the eye (the rear, or posterior, element) is definitely composed of multiple lenses.

Note the large size of the anterior optical element.

Note the large size of the anterior optical element.

That the shield serves as both an aperture and heat shield so that the heat generated by the intense light required for projection does not damage the optical elements of the objective seems obvious. One may also feel assured that the large diameter of the lens elements owes to the need for the objective to provide a very bright image. For a 10x objective of such advanced age to be composed of quite so many lenses at first seems strange until one considers the need for the objective to provide an unusually flat field of view without any assistance from intermediate lenses or compensating oculars.

Its Use

In its day the objective likely was used with combustion lamps. Intense sources such as the limelight or Wellsbach gas mantle may have been employed when projection was required over any significant distance. For photography or short projection it may have been possible to use a well-condensed oil lamp or cut window shade so as to permit the use of daylight in an otherwise darkened room. The objective may have continued to see service after the commercialization and wide availability of electricity brought such options as the carbon arc lamp and incandescent flood into use, but one may only speculate.

The use to which The 1 Inch was put is well known, that use of course being projection. Consulting an early edition of Bausch’s Manipulation of the Microscope provides that a projection objective should be used at a projection distance of 15 or more feet. Gage, Carpenter, and other authorities, recommend projection objectives for micrography either alone or with suitable oculars. One may be assured that except in extraordinary circumstances no one was making micrographs enlarged to the extent had at a 15 foot distance, so that the use of the objective for smaller projections is suitable.

However it was employed, one may be certain that The 1 Inch saw a significant amount of use. Projection was practiced rather more often in the early days of microscopy and for some the method was so frequently called upon that it was not adequate to merely rely on a simple achromat. Throwing an image upon a screen across the room was, and remains, no mean feat for any (excuse the insult) humble microscope.

∗Likely, but by no means certain. Methods exist for determining the optical and mechanical tube length of an objective for which these are not known, at some later date these will be treated with.

†For details on the calculation of power from equivalent focus please see my earlier entry on the matter available here. The link opens in a new tab or window.

Next time: The 1 Inch in action! -K

Projection Microscopy I

There are Objectives and There are Objectives

For anyone who follows microscopy in only it’s modern form, it might seem as if it’s all been figured out and objectives are as fitted to the stand and needs of the microscopist as a key is to a lock. For those who have a few different manufacturers microscopes in the lab it might be more apparent that it is not all figured out; some favor DIN standards or the classic RMS standards, this stand requires infinity corrected objectives while that does not. If the microscopes is “of a certain age” one might be distressed to find that its objectives are par-focal for odd lengths or incompatible with lenses of an earlier or later vintage. Knowing a little optics one will quickly become aware of the importance of using the proper objective for the stand to obtain the best image.

When considering the genuinely antique microscopes one might at first find things more rather than less confusing. Manufacturers of one line or another included different portions of the microscope body when measuring for tube length, in essence quietly adopting different tube lengths than that marked upon the objectives barrel. Cover glass thickness was as variable then as now and so were the thicknesses for which objectives were corrected.

But if the standards to which an objective was built were less, standard, so too was the stand more fluid. Oculars were never integral, tube length was variable, often enormously so, and the microscopists knowledge of their instrument was nearly important as the instrument itself. It can truly be a daunting task to search out the components necessary to outfit an antique microscope be those items hidden in dusty shops or dark attics. More daunting still may be the hunt for the information required to use the latest find effectively. Permit a weak attempt to throw some light on one dark corner.

Projection Objectives in Theory

Most are familiar with achromatic and apochromatic objectives, with oil or water immersion, even the specialized Homal photomicrographic objectives, but the projection objective was an entirely different animal. Designed specifically for micro-projection the objective had several difficulties to overcome. These difficulties will be familiar to the modern worker, however much the method by which they are solved has changed.

Projection adds a variable that is an established constant in normal microscopy, projection distance; the distance at which the projected image forming rays are interpreted by the viewing surface. While the objective provides the magnification of the object, the projection distance is responsible for enlargement only; a magnification of ten diameters is a magnification of ten diameters be the image an inch across or a foot. All of the visual information that will be present in the projected image is resolved (added) by the objective. The enlargement serves only to act upon what is already present and might be of any size depending on the need of the microscopist.

The projected image increases in size as the distance at which the projection is interpreted is increased because the light which exits the optical system is diverging. As the light diverges a constant amount of illumination is asked to light an ever larger area. Naturally, one can not expect to light up a drive-in theater screen with a classroom film-strip projector; so too one should not expect to set an image five feet high on the far wall of the room with the usual sort of illuminator. The projected image will become dimmer as it is enlarged and any aberration will become more apparent.

Because spherical aberration increases at the edges of the field of view, it was frequently practice to simply limit the field of view to provide a restricted but more perfect image. Spherical aberration might also be corrected by the introduction of additional component lenses which would entail an attendant decrease in brightness, just as with chromatic aberration. Spherical aberration and chromatic aberration are two difficulties that all objectives strive to overcome to a greater or lesser degree. Doing so while maximizing the amount of light which enters (and therefore exits) the system is a similarly common goal.

One might question why then a projection objective should be any different from a standard objective. In normal (non-projection) objectives the entire optical system was considered and constructed so that the virtual image viewed by the microscopist is as perfect as might be for a particular need. For projection microscopy the real image is the one which must be rendered the most perfect and the equations of the optician altered accordingly. Projection objective would often be employed alone, without the addition of an ocular all of which must be accounted for in its construction.

In any case the important thing to remember is that normal objectives work with the microscopists eye as the final component of their optical system, for projection objectives that final component is the screen upon which the image is thrown.

Still with me? Don’t worry this will all get more fun shortly. -K

Elucidating Illuminators: I

Integrated Illumination

The choices one has in microscope illumination are often, of late, made by the microscope manufacturer. That choice is not always the one that the user would make for themselves, or even the best for general use. In general one might say that with integrated illumination, whatever the quality, age, or price of the microscope, one can rely on prompt success when it comes to simply obtaining an image. The perfectness of that image may range from excellent to abyssal, but something will certainly be seen at the eyepiece.

Today, if one is possessed of a microscope with integrated illumination little need be done for acceptable operation. A microscope equipped with a mirror is as easy for general, but more complex for critical work, when compared to a stand with integrated illumination. The mirror microscope is next to impossible to use at its full ability without a measure of effort and understanding of some of the methods of illumination for transmitted light microscopy. Let us begin by looking first towards a number of microscopes equipped with integrated illumination of various sorts and speak to the merits and abilities of each.

Köhler Illumination with the BalPlan

IMG_1656rotatecropeditThis model of the Bausch & Lomb Balplan uses a halogen lamp mounted in the base to illuminate a ground glass Plano-convex lens secured in the base. From there, a beam is sent through an iris diaphragm and fixed focus lens towards a right angle mirror which sets the light vertically through a second lens. The coherent light which emanates from this lens is then passed through the microscopes rack and pinion 1.25N.A. aplanatic condenser. The condenser is equipped with an auxiliary lens which may be swung into place to expand the pencil of light and provide a large enough light source for use with a low power finder objective.

With an object in place (and in focus) it is the work of a moment to obtain a focused image of the iris of the illuminator using the condenser, and adjust the iris of the condenser to so that the numerical aperture of the condenser matches that of the objective for true Köhler illumination. The method of August Köhler to provide a field of light perfectly uniform and out of focus, while retaining all the characteristics required for maximum resolution are today recognized and accepted as optimum for advanced work. Köhler illumination puts a focused image of the light source (the filliament of the lamp or grain of a ground glass screen) at the iris diaphragm opening of the substage condenser and again at the back focal plane of the objective. A bright evenly lit field is then formed at the iris diaphragm opening of the illuminator and in the image plane of the focused microscope slide.

In the photograph the illuminators iris diaphragm has been stopped down well beyond what would be used in practice to better show the in focus diaphragm simultaneous with the image plane of the specimen. Note that despite the light source being a ground glass there is no visible grain in the photomicrograph.

Critical Illumination with the AO Fifty

IMG_1658rotatecropeditThis American Optical Fifty series microscope makes use of an uncomplicated 15watt (medium base) incandescent bulb as the light source. Mounted in an adjustable housing in the microscopes base the light of the bulb is sent first through a blue filter. Immediately above the filter is mounted a single short focus Plano-convex lens with a ground glass flat surface. This arrangement makes up the integrated illuminator common to microscopes of the Fifty, Sixty, One-Fifty, and One-Sixty series. Above the field lens of the illuminator is a 1.25N.A. Abbe condenser on rack and pinion adjustment.

With this setup Köhler illumination is not possible. Instead a less perfect but far less mechanically demanding type of illumination known as critical illumination is possible. Prior to the modern acceptance of Köhler illumination as the gold standard it was practice to work with a large light source and less complex illumination apparatus. Rather than a focused image of the light source, a brightly lit field is formed at the opening of the substage iris diaphragm and the back focal plane of the objective. Unfortunately, this results in a focused image of the light source (and all its attendant grain or irregularities) being formed in the image plane of the specimen. The worker is then apt to throw the substage condenser slightly out of focus so as to avoid the distracting image of the filament when not using a sufficiently large homogeneous light source.

Not that in the upper photomicrograph there is evidence of dust on the ground glass illuminator, as shown by dark areas indicated by the pointer. In the lower photomicrograph the substage condenser has been thrown slightly out of focus to prevent grain and dust from appearing in the image plane of the specimen. In general use one may chose to rack the condenser up or down if the grain of the illuminator is found distracting, there is however a slight increase in spherical aberration and decrease in resolution.

Add-on Integrated Illumination

IMG_1657cropeditLargely as a response to the need for simplicity and convenience when introducing microscopy to students, a large number of manufacturers began to sell small self-contained light sources in the mid-twentieth century. These simple lamps could be used in the conventional way with a mirror bearing microscope by sitting the lamp upon the table, or by mounting it directly to the base of the microscope. The Bausch & Lomb lamp shown here was available with two different mounting brackets; one which made use of the mirrors mount, and that pictured which would be fit over the removable substage iris diaphragm before it was screwed back into the base of the substage condenser.

This illuminator consists of a small 15watt (medium base) incandescent bulb in a switched, sturdy, Bakelite shell. Over the bulb is mounted a blue glass filter with one ground glass surface, followed by a Plano-convex lens. The exposed convex surface of the lens is ground as well. Depending on the arrangement in which the illuminator is used, one can work with critical illumination or not, but again Köhler illumination is not possible.

Such lamps are chiefly recommended by their simplicity, durability, and availability at low cost. Less often mentioned is the versatility of such lamps. Any illuminator of this sort§ may be arranged to provide ample light in most circumstances a beginning microscopist is likely to encounter. It may be used with or without a substage condenser, or even without the mirror; as it may be set on the table between the foot of the microscope with the light projecting upwards through the stage. If such a light source is to be used it should be considered a component of the microscope, and kept with it at all times.

In the upper photomicrograph the lamp was placed on its back below the substage. Although the illumination is not critical the central portion of the field is even lit and free of grain. In the lower photomicrograph the lamp was positioned ten inches from vertical axis of the microscope and its light reflected by the plane mirror. With the surface of the ground glass focused critical illumination resolves the center of the field of view with slightly less spherical abbaration, although grain is once more introduced into the image and it is evident that this author did not take time to properly center the lamp.


∗Well into the twentieth century arguments continued to be raised as to the technical or practical superiority of the various methods of specimen illumination. One may at first wonder at the controversy but it seems to have largely resulted from the need for more specialized light sources with Köhler illumination and the practicality of critical illumination when used with any large (flame or cloud for example) light source. The past popularity of point light sources is a product of the growing pains of the adoption of Köhler illumination as it was generally felt that a bright point of light provided more capacity for filtering without diminishing intensity.

†Technically, in critical illumination that brightly lit filed is formed just before the back focal plane of the objective and moves closer towards it as the power of the objective is increased.

‡Obviously with the lamp mounted directly to the substage condenser one can not bring the ground glass surface into focus in the specimen image plane as is required for critical illumination. Working with the lamp on the work table, or with the lamp mounted to the mirror bracket it may be brought into focus with the substage condenser if it is in range of the primary focus. I am uncertain as to why B&L would grind the convex surface of the lens, but I suspect it was motivated more by the desire to make the device less open to damage by unskilled hands (there are no exposed polished lenses to worry about) than by optical concerns.

§All of the major manufacturing houses put out similar lamps with differing arrangements as to bulb and glass. A popular American Optical model was equipped with a blue filter and swing out ground glass or bullseye lens, providing a limited means of controlling the intensity of the light. Modern LED or incandescent lamp housings are available that will mount in place of the double sided mirror of most vintage stands, one need only verify that the mounting is compatible.

If things seem complicated don’t wait on the availability of fancy equipment, great work has been done with desk lamps and white clouds. Next time we’ll look to external illuminators. -K