The Bausch & Lomb Model R part: III

The question of the determination of a microscopes magnification has a distinct tendency to be treated in either a profoundly technical way or only the most basic terms, never mind the source. On the simpler end of things it’s often put similarly to this: the magnifying power of a microscope is determined by multiplying the power of the objective by that of the ocular. Well, lovely. That certainly buttons that up doesn’t it, no? There may even be a few lines here of there on the power of an objective or ocular but all such texts take it as given that the optical components will be marked. At the opposite end of the spectrum one will find page after page of complex optical formulae and jargon like principle poster focus and Ramsden disc. Fortunately those formulae that are printed can be made rather more meaningful to most people by simply substituting words for symbols, as such:

Magnifying Power = Tube Length x Distance of Distinct Vision / Focal Length of Objective x Focal Length of Eyepiece

Which is great, if you want to muck about in physics class and measure the focal length of your lenses. One could of course forego that in favor of a little bit of basic math, if one had an eyepiece micrometer and an object micrometer, but then the Model R uses non-standard diameter optics so the chances one has an reticule of the right size for the narrow ocular is slim, and in any case it’s a closed system-not something one would easily disassemble. So what if you haven’t got anything, not even a stage micrometer? I mean the Model R was made for kids right, what kid just happened to have a hankering for a stage micrometer first thing when they got a microscope? Alright, maybe a lot of us did, so we’ll use one but bear in mind we can do this with any object that has a known diameter, like a human red blood cell (7.2 microns at the widest point) or a human hair (in the neighborhood of 70 microns is diameter.

All the physics used to determine magnification is well and good but pales as a practical exercise for the microscopist to comparing the known size of a particular object to the magnified size of that object. In order to do that with math one needs to know a great many things about the lenses to begin with, most of which is best suited for classwork in physics only. In order to make the same comparison in an almost exclusively practical way one need only set up the Model R (or any microscope) as below.

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  1. Place the object of known size (be it a blood smear or stage micrometer) on the stage and focus the microscope.
  2. Incline the joint so that the microscope is horizontal The Model R hasn’t got an inclination joint but the body and stage comes off from the foot and may be mounted horizontally.
  3. Use a rule to position the exit pupil of the microscope 250mm from a sheet of paper taped to a wall or other support.
  4. Position a bright, high intensity light source so that it may be focused on the specimen from below the substage.
  5. Turn out the room lights.
  6. Mark the locations of several divisions of the micrometer or a few red blood cells on the paper.

Now that the paper has been marked only one further measurement is required. The marks made by projecting the specimen on the paper are of a known division. They are also of a size that may be easily measured with convention means.

  1. Use a rule marked in millimeters to measure the divisions marked on the paper.
  2. Line up carefully and note the number of divisions on the paper that are needed to span the distance perfectly between any given number of either.
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Yes, I did chose to awkwardly lean over the entire setup rather than walk to the other side of the table!

Now for the math, in this case the formula is much simpler than one might expect. One need only divide the distance as measured on the ruler by the known measurement of the magnified and enlarged divisions marked on the paper. Therefore if the divisions of the stage micrometer are 0.01mm, and at the Model R’s most powerful magnification (draw tube fully extended) they measure precisely 4 divisions in 12mm the formulae would be 12/0.04 = 300 diameters of magnification. It’s pretty nice to see that that confirms the marking on the draw tube. Repeating the process with the draw tube fully retracted one finds that 2 divisions as marked on the paper span 3mm exactly, 3/0.02=150 diameters of magnification.

With this knowledge one can accept that the marked powers on the draw tube are accurate, but that doesn’t inform on the individual power of either the objective or ocular. One will of course recognize that removing the front element serves to reduce the power of the entire system by half as that is what the markings indicate. Unfortunately this does not enable one to know the power of the individual elements. One need only repeat the process without the ocular to find the power of the objective alone. It then becomes a simple matter to know the power of the ocular, power of the entire system / power of the objective = power of the ocular.

Repeating the steps above, except to this time measure 250mm from the rear of the objective lens provides the following measurement. Twenty divisions (marked in intervals of 5 each) measures 4mm on the paper. Such that, 4/0.2=20 meaning the power of the objective is 20x and the ocular is therefore 15x which further indicates that removing the front lens element reduces the power of the objective to 10x.

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The Bausch & Lomb Model R part: II

It’s time to see how the optics of the original B&L Model R perform. To begin with a few common test objects, beginning in this instance with the wing of a fly, will be evaluated visually. Then, the next visual test will be the scales of a Podura springtail followed by the more finely striated Pleurosigma angulatum. The ability to resolve the features of these objects once served the same descriptive needs that the numerical aperture measure serves now.

It’s worth mentioning that no special effort will be made to ensure perfectly clean elements, so although this microscope is in exceptional condition for a stand of its age one could expect to see a not insignificant improvement in resolution should the effort be made. All photomicrographs taken will be made with a ring-stand supported iPhone camera. To avoid putting up an over abundance of images that will will only be given a cursory place only two images of each test object will be provided. The lowest power is that marked on the Model R microscope as 75 while the highest power is that marked as 300. It’s worth noting that to configure the Model R to provide the lowest power one must remove the front element of the objective lens and set the draw tube to its shortest length. To achieve the highest power one must use both elements of the objective and set the draw tube to its greatest length. Lighting is provided by a high variable intensity condensed B&L illuminator.

In the low power photomicrograph we can see that there is some indication of markings on the various cells of the wing and make out a pattern of hairs on the costa. At the high power end of things we can clearly see the individual hairs of the costa, they are not the toothy spikes represented on the first photomicrograph. The pattern of hairs on the marginal cell is clear but the individual definition is somewhat obscure. It is not immediately clear that they are in fact raised hairs, easy enough to determine with a bit of back and forth of the focusing knob. All things considered the imaging abilities of the microscope are surprising. The axial third of the field of view is surprisingly clear and sharp, even when viewing so comparatively thick a specimen as a fly wing. Except at the lowest power, without the complete objective in place, there is very little evidence of chromatic aberration.

In the low power image of the podura scales we are able to discern some implied texture on the surface of the scale, it’s obvious it is not simply a color gradient. Again it is clear that there is some chromatic aberration, more in evidence due to the slight misalignment of the illumination source, but one should expect that when using a divisible objective. Turning to the high power photomicrograph the texture becomes a clear pattern of lines although, one could certainly not make any secure judgment as to the nature of the texture in cross section. It should perhaps not be surprising that the Model R can make no clear statement on that count, there was in fact profound disagreement on the form a podura scales texture would have that was unsettled until the rise of electron microscopy. As a side note, these scales are not intact lined but rather feature a pattern of lines composed of fine hairs.

Although the Model R did not provide the performance of a professional stand on any of the test objects thus far it certainly stood tallest on the fly wing. It’s then somewhat pointless to set it against the Pleurosigma angulatum as there’s little reason to expect it to resolve the finer points of the diatoms test. Fortunately, one would hardly expect someone to put such a difficult object before the Model R. It is gratifying though to note that if hard pressed one could certainly enjoying viewing diatoms with the stand, clearly it’s not the ideal object one might chose to observe but it certainly performs far better than one might expect. The red and blue fringes of an achromatic objective are obvious on the photomicrographs of the diatoms; they are by no means reason to disregard the Model R. It would be a great thing if a microscope of this quality were put before every elementary school student rather than the sort of needlessly complicated and overly ambitious toy one can find at any of the “science” themed stores that exist in shopping malls and digital marketplaces.

In part III, a few simple tests one can perform to find the powers of the optical elements. -K

The Bausch & Lomb Model R part: I

The microscope is something of an emblem of science. Unlike similar fetishes of science such as the test tube, the microscope stands as well as a banner for the curiosity of scientific pursuit. It’s an amazing thing and has had an impact on the sciences similar to that the automobile has had on transportation. Fortunately, while the automobile exists in a state of age restricted licensure, the microscope is available to all-or is it? What about price? What about the availability of something serious but attainable? What of something in between a fly swatter and an atom bomb?

Over the years the optical company Bausch & Lomb has broken new ground any number of times in the world of microscopy. Growing up outside Rochester, N.Y. it was something of a given that the microscopes in school would bear that prism-shaped logo and great treasures would be hidden away in the attics and sitting rooms of my childhood. These were all, however, great heavy things of cast iron and brass, replete with fragile glass elements and cases bigger than any bread-box. There was something else though, something different, and it started with the Model R.

Sold in the 1930’s during the years of the world-wide economic downturn popularly called “The Great Depression” the Bausch & Lomb Model R had a retain price of twenty-one dollars in the United States. Depending on the precise year one elects to compare that equates to anywhere from $300.00-$400.00 in todays currency. This was a time when the average factory worker was earning some $0.40 an hour and could expect a weekly paycheck which would only just cover the cost of the Model R. It was accessible but surely out of range of anyone without the willingness to sacrifice to obtain it.

The styling of the Model R was such that considered on it’s own it would not seem out of place beside a professional instrument from its day. Black enamel paint and bright nickel plating were the norm. The look is replicated in the Model R with a glossy black Bakelite stand and shiny metal body tube. A few points that quickly stand out are the lack of an inclination joint, the single focusing knob and total lack of a fine adjustment.

Then there is a draw tube… What? No! To dispense with the need for a nose piece and multiple objectives, a pair of which would cost more than the entire Model R, this microscope made use of a varifocal lens system (rather like but distinct from a zoom lens system). With this simple change the microscope was enormously reduced in price.

While today one might expect a plastic snap-case if any were provided at all it’s well to remember that in the 1930’s when an item came with a box it was as likely to be wood (perhaps more so) as pressboard. So if the look of the Model R were a product of it’s time, what can be said of it’s utility?

For that, watch this space for part II where the Model R will be put through it’s paces -K

Opaque Object Microscopy part: IV

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This last installment will be a look at the B&L STM Electroplater’s Microscope. A refinement on the Standard Teaching line, this Metallurgical microscope is designed for simplicity and easy of use. Superficially it resembles any of the other stands of the ST line, enamel grey with an integral, two-position nose piece, it manages to provide all that is needed and nothing that is not. A single intensity transformer powers a Nicholas style light source that is fixed to the side of the stand just above the nose piece. The light path features a filter holder but is not equipped with either field or aperture diaphragm, rather the light path is permanently restricted to a degree appropriate for the two supplied objectives.

3ca50003-f562-435a-95e7-fbda76ccfc37-649-000000960bbb8e15_fileEach objective is of 215mm tube length construction and is corrected for use without any cover glass. One is a low power finder (5x) and the other a higher power (40x). One will quickly notice that neither is of the power one expects to find on a student microscope; 10x and 43x being the usual combination. The reason for this quickly becomes apparent when one calculates powers in consideration of the characteristics of the stand. A 215mm tube length, correction for no cover glass and the nessecity of a short working distance. At right one may see the working distance of the 40x objective.

In place of a standard eyepiece it features a filar micrometer ocular as for most all metallurgical work one would desire the ability to take measurements. In that same vein the fine focus adjustment is graduated as well. Although the stand features an inclination joint, there is no adjustment for the stage and no arrangement to provide for transmitted light work. Internally, the light source is directed downwards by a half silver mirror which is not adjustable without tools. This was no doubt done so that it may be aligned once and may then be employed without a further thought to the matter. When I first acquired this stand the reflector was shattered. No matter, it was easily replaced and enabled me to have this delightful little stand for no more than $40 USD plus shipping!

In the interest of providing a little eye-candy I placed the seemingly polished brass surface of a pocket knife in a bit of putty on the stage. Putty or a specimen holder of the standard sort is recommended for most specimens for the sake of stability. A photomicrographic ocular of 7x power and a Pentax microscope camera adapter was used to take the photomicrographs.

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Pentax Microscope adapter in place on STM Electroplater’s Microscope

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Low Power

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High Power

In the above images we may observe the compromises made in the STM Electroplater’s Microscope. The low power photomicrograph shows that the apperture size employed in the illumination system is such that slightly less than the entire field is evenly illuminated. In normal visual use with the 10x filar micrometer eyepiece this is only very slightly noticeable. The 7x photomicrographic eyepiece exaggerates the defect because of the larger field of view it provides. It would be possible to artificially crop out the uneven area by using a greater photomicrographic tube extension, but the one I use is perfectly sized as to be parfocal with the eyepieces when employed on a trinocular stand.

In the high power image we will immediately notice how the curved portion of the specimen surface falls out of focus and quickly devolves into a mess of chromatic aberration. This is why flat surface polishing is such an important part of metallurgical microscopy! Luckily enough, the (exceedingly self-congratulatory) microscopist managed to place the specimen such that the two larger scratches (dark marks slightly left of center in the low power photomicrograph) remained in the frame under the higher power objective. Note how the formerly dark high contrast scratches are now fully illuminated and visually interesting.

Next time: Adventures in Large Format Photomicrography! -K

Opaque Object Microscopy part: II

Before looking closely at microscopes which were purpose built for reflected light work it is first imperative that one understands the requirements of such a stand. It must of course allow for some means of illumination, but beyond that there are a few needs that are not so obvious. Consider the compound transmitted light microscope, several aspects of it’s construction are dictated by the optical properties of human vision, a substantial number of others are dictated around changeable constants that are functionally arbitrary. A microscope slide and cover slip that is of a given thickness greatly simplifies the construction of an optical system that will provide an ideal image with minimal and known defects. Furthermore, it dictates that all specimens will be of a consistent and narrowly variable thickness.

Any microscope that needs to accommodate opaque objects will either have to account for the need to examine specimens of unknown thickness or be considered specialized. It need not be overly complex, one could make use of a stand having a significant range of motion in its coarse focus, or possessing a means of modifying its base working distance-as one finds on many stereo microscopes.. A further option would be to articulate the stage such that it may further enlarge the accommodation of the coarse focus. This is a simple mechanical alteration to an otherwise standard microscope foot; as a condenser is unnecessary, the stage is for all intents and purposes mounted to the condenser mount. An inverted microscope forgoes this need entirely by radically re-configuring the entire apparatus, a good investment if only one microscope is liable to be acquired, but it’s worth noting that inverted microscopes are in general far less common on the second hand market and consistently more expensive.

It is also required that the microscope provide for the specialized optics of a reflected light system, namely the light source. In the initial post we saw the difficulty of using a light source external to the image forming optical axis. It is therefore required that the illumination system be congruent with the optical axis. This requires that there will be some additional apparatus placed somewhere in the optical axis, by convention it is generally placed outside of the body, between the nose-piece and the end of the body tube. Whether this is dictated by optics rather than mechanics (or the economics of manufacturing) is unimportant, the result is the same.

At a point in the optical column of the microscope a high intensity light source is introduced. In every example of which I am aware this light source is situated perpendicular to the axis. It is suitably condensed and often fitted with a pair of iris diaphragms (a field diaphragm and condenser diaphragm) as well as filter carrier before being directed down towards the objective via a reflecting surface. A prism or half silver mirror is the usual method; often both are available with the ideal choice being contingent upon specimen and objective.

The actual construction of the reflector is related to the properties of the objective, with all parts involved being of a number of mechanical types. All of the differences in the system of illumination are chiefly concerned with the path of light. There exist two primary types: coaxial and vertical. It’s confusing because few operators, and even manufacturers are careful with their terminology.

Both coaxial and vertical illumination are methods of reflected light microscopy, and coaxial is by definition vertical while vertical is not necessarily coaxial. Coaxial illumination is so called because the path of the light source shares its axis with the path of the image forming rays. The poor mans coaxial illuminator is a flashlight held to one eyepiece of a binocular stand while the other is used for viewing-authors note: don’t do it! The axis of illumination and image formation are one and the same. Vertical illumination is a story of two axes where each is distinct but parallel. The most common type of vertical illuminator is able to provide both methods, but the quality of the coaxial illumination is often inferior when compared to modern outfits designed for coaxial illumination.

Without bothering to get in to dark-field (yes, there are dark field objectives for reflected light work) there are two types of objective one will encounter. The first is essentially no different from any standard objective, excepting of course for differences in the common powers, corrections, and other properties best left for later. The second, and more complex type, is designed to work with a particular type of light source. This second type (which I have always thought of as metallurgical as that is how the BalPlan microscope line of B&L designated them and they were the first I used) caries within its body a transparent glass bushing which extends from the mount to the object lens, surrounding totally and supporting the lenses of the objective. This glass pipe is little more than a means of placing a ring of evenly diffused light on the specimen in a place where the objective itself would obscure other sources. Properly arranged, it is an excellent system and dispenses with much of the glare one will find in poorly aligned coaxial or even vertical systems.

There, next time: photos! -K

Opaque Object Microscopy part: I

I think at this point I’ve been away long enough that this may qualify more as a return from the dead than a return from a hiatus. -K
Most of what anyone at the level of a hobbyist is going to be looking at with a microscope is going to be what is convenient. Now, this is not meant as an indictment, merely a truthful commentary. For the bulk of those with a microscope this is going to mean that what one is going to be looking at is dictated first, by the microscope which is available, and only after by ones interest. When conditions allow this translates to transparent objects for the compound microscope and large opaque objects for the stereo microscope. There are, thankfully, limitless opportunities for the indulgence of ones interest regardless of the microscope which is available.

The next few posts will focus on a category of microscope which is rather less common but is specialized for a particular variety of specimen. The particular type of microscope is rather less common, and one could speculate endlessly on the reasons for this. This microscopist is of the opinion that the reason for this is in general attributable to its being far harder to prepare specimens for a reflected light microscope, than to settle for lower magnification and use a stereo microscope. However, there are a variety of applications for which one will find the power of a stereo microscope lacking. One is then left with the prospect of attempting to so treat the specimen as to render it suitable for transmitted light microscopy, or of finding some way of providing suitable lighting and using a standard compound microscope. Anyone who has attempted to observe an opaque object at high power will understand the difficulty of providing for adequate illumination.

For the sake of completeness, here are the logistics when one is forced to make use of a standard compound light microscope for reflected light work. One might first make use of some small and high intensity light source, employing it in such a way that the termination of the visual optical axis is brightly lit. This is actually surprisingly simple in the present day when an LED flashlight the size of a shotgun cartridge is brighter than any oil lamp. After oil lamps gave way to electric lamps the microscopist was required to somehow retrofit a standard lamp bulb so that it would provide a bright beam of light with no errant brightness.

There was also the possibility of purchasing a small bright light source such as a Nicholas lamp. Although those were surprisingly expensive in their day, they are quite economical now and widely available second hand provided one is willing to type a few searches. Before getting to much farther off topic, do consider picking up a Nicholas illuminator. Color temperature aside, the tight beam is well corrected and the arm most are fitted with is a great asset. Working with a Nicholas lamp and a compound microscope, we will quickly see why this method is far from ideal.

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Poorly set up.

In the above image we can see that a light source is set up sufficient to permit the specimen (here the engraved body of a pocket knife) to be brightly illuminated to the naked eye. There is enough working distance that no significant difficulty was involved in setting it up. A quick look through the eyepieces will immediately demonstrate that this set up is not only far from ideal, but entirely unsuitable. The light source is a painfully bright halogen bulb but the view from the oculars is quite dim, contrast is excessive, and there are visible color fringes even though the lowest powered objective (here a 30mm EF/3.5x achromat) is being employed. Some of these defects can be corrected even in this compromise set up.

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Ideal compromise set up.

In the above image is shown an exaggerated ideal arrangement of the available resources. If one takes the stage for a plain, and draws a vertical line along the visual optical axis the light from the illuminator should be arranged so as to be as at the most acute angle to the optical axis possible. This will go a long way to limiting the extremes of contrast and removing the color fringes. It will similarly  render the specimen as bright as possible under the present conditions.

It may not be immediately apparent but working distance is the limiting factor here. As soon as one moves beyond the 10mm or so that one is afforded by a standard 16mm (10x if you’re more comfortable with magnification than equivalent focus) objective the few working distance of a 4mm (43x) objective is far to short, even that of a rather less common 8mm (21x) objective will be much to short for all but the most intrepid of operators. Then, should illumination be secure one will be presented with a view of such poor quality that the effort is entirely wasted.

Next up… vertical illumination.

Another Entry Level Microscope

The Bausch & Lomb Standard Teaching (hereafter ST) microscope is hardly the finest instrument one may buy, though it is a true and reliable stand. Carrying on the great tradition of simple and rugged construction begun with consumer microscopes designed for both student and arm-chair scientist the ST is descended from the FL (For Learning) of decades prior. For a look at a far earlier version see this previous post.
Immediately recognizable by its B&L slate-gray enamel one will always know an ST or FL upon sight. Telling one from the other is simple as well, meerly glance at the fine focus. If it’s located at the inclination joint it’s of the ST line. A fine focus on the arm above the joint is indicative of the FL line. There are a number of less immediate means of identification, the contruction of the foot, the mechanics of the coarse focus, the presence or abscence of a focusing stop, and the finish of the stage, to name a few. This is about the ST though, so on with that.

As with most B&L lines there are more variations on the basic ST stand than one would expect ranging through optical components to convience features. Apart from the regular ST there is also the Inclined model (IST) which carries a two piece body tube with prisim adjusted angled ocular tube. It’s called the ST for a reason though and the versions that could be had ranged from the very modest too three objective, condenser equipped models with mechanical stages suitable for entry level bacteriology, and other oil-immersion work. The most advanced models made use of the same objectives as the flagship Dynoptic line, their compromise was the slide focusing condenser. A slide focus condenser is not to be sneared at, but when used with an external illuminator one may be hard pressed to secure Khler illumination. Pressing a simple substage to its limits and restricting the user to a monocular system the advanced end of the spectrum is not where the ST shines.

At the lower end one finds the ST in its element. The not quite there, complecated yet inexpensive condenser set up is replaced with a simple rotaing disct diaphragm and either concave mirror or Optilume lamp. It’s worth mentioning that B&L with the ST line once more holds to the concave mirror only (for condenserless substages). The Optilume is well suited too, offering a bright field of view and on/off simplicity.
Perhaps the most laudable feature is one that more advanced users will never make use of. The ST microscopes take seriously the inexpert focusing of the student or novice microscopist. However much one drives home the importance of the microscopists obiesence there are those who will insist upon obtaining coarse focuse with eye at the lens, all such marksmen should be forced into carears as snipers. The ST line repesents the first standard Prefocusing Gage from B&L.

This simple addition has doubtless saved more than a few slides from the hazards of crashing objectives. The Prefocusing Gage is a humble projection from the body tube through which a theaded hole has been bored. A hexagonal set screw is fitted through the hole and a smaller flat headed set screw penetrates from the side to lock it in place. In use one need only to place a slide between the gauge and arm and gently rack down the coarse focus until contact is made. Removing the slide to the stage one will then find the specimen in rough focus and need only touch the fine focus knob until examination is complete. As with most every microscope of quality the objectives are parfocal so the utility of such a device should be obvious.

If one has other stands at their disposal one may see quickly where costs have been saved on the ST microscoscopes. The fine focus mechanism is mechanically identical to that of the Dynoptic but there is not graduation upon the adjustment. The body tube has been cast from a single mould so that nosepiece and ocular tube are permanently in place. The stage likewise is cast as one with it’s mount and may not be exchanged without likewise replacing the entire substage.

For those people out there who have and use an ST or IST here are some useful measurements from the manual (I’ll get around to uploading a scanned copy of it sometime).

First the table of approximate equivalent size of the pointer in the plane of the specimen:

Then the table of magnifications with the same 10x Huygenian eyepiece:

What Happened to Spring?

Here we are enjoying the second week of the spring season and yet again, it’s snowing. -K

Spring is easily among the most productive seasons for the nature microscopist. Freshwater diatoms are at their peak in the spring, and many insects that have overwintered are out, as are the various forms that hatch as the weather warms. If there is a body of water a convenient distance from ones base, it can be very educational to chart the rise of spring by each day doing a quick survey of the contents of a drop of water. Does the density and variety of microbes rise steadily with temperature? Does it fall or continue to rise as snow-melt dilutes the body of water with a seasonal influx? Or does it just keep snowing!

Note the artifacts around the arrow.

Note the artifacts around the arrow.

Above is an image taken of a preserved snowflake. It is not a very good preparation but illustrates a couple points that should be emphasized. Firstly a bit of information on the preparation itself. Snow was allowed to fall on a clean slip and over a likely specimen was dropped a solution of polyvinyl formal. Without the addition of a cover glass it was maintained at a freezing temperature for a few minutes as the resin formed a cast of the captured ice crystals in a gas permeable clear plastic. When brought indoors the water from the ice crystals was able to evaporate leaving a hollow shell that may be observed.

At the tip of the arrow we observe a large air bubble. As a result of the method of preparation the air bubble formed because of air trapped in the resin itself. The large size of the air bubble makes its identity obvious and more of a nuisance than a distraction. To the left of the arrow we see an additional bubble much smaller in size. Such bubbles will spoil any preparation but illustrate an important lesson concerning depth of focus and optical alignment very well.

In the above image the small air bubble presents with a distinct black outline. This black outline is too large to illustrate diffraction rings well, but that is the principal behind much of the following. The black portion is caused by stacked layers of out of focus image planes. As a result of the known shape (spherical, with some distortion) of air bubbles we can easily picture it in cross section, the optics of out microscope can not however provide a clear outline of the section because the layers above or below obscure the image.

The top aspect of the bubble in focus.

The top aspect of the bubble in focus.

Increasing the magnification we can easily bring the top aspect of the bubble into focus. The black outline then becomes an aid in determining the actual form of the bubble. If our lighting is truly central we are able to discern that the portion of the bubble nearest the pointer is thicker than that which is farther away, as evidenced by the more pronounced black fringe.

If our lighting is not central then we can use the bubble to determine in which direction to move our condenser to bring it into alignment. In the above example we might take the lighting as generally central, which we can tell by observing the black fringes on the larger air bubble. However, to be truly accurate we can observe that there is a very light fringe just inside one edge of the larger bubble. Adjusting the condenser a very minor amount in the direction opposite that fringe we are able to obtain more perfectly aligned illumination.

Our condenser properly aligned provides axial lighting.

Our condenser properly aligned provides axial lighting.

Although still not entirely axial we observe that the fringe is much less in evidence. Properly aligned illumination is vital to correct interpretation of the image. We can now form a more accurate mental image of any object we elect to observe simply by sending the fine focus just above or below the image plane we wish to observe. Axial lighting provides that any dark fringes we observe will be a product of the specimen rather than an artifact of our illuminating system.

Determining Objective Magnification

Here’s hoping this is useful as more than just an academic exercise. -K

In the previous post we looked at equivalent focus as it relates to the power of an objective. It was noted that the power marked explicitly on ones objective is sometimes at odds with that implied by the equivalent focus. Today we’ll look at one way to determine the actual power of a given objective. The method used is among the more equipment heavy, but it is also one of the least demanding so far as manipulations go.

One will need the following:

  • A microscope with a draw tube (or an eyepiece collar)
  • A 10x Huygenian eyepiece
  • A 10x Ramsden eyepiece
  • A stage micrometer
  • An ocular micrometer (installed in the Ramsden eyepiece)

Method

Using the Huygenian ocular, with the stage micrometer as an object, and the draw tube set to the length for which the objective is corrected (160 in most cases) the objective to be measured is brought into sharp focus. The Huygenian ocular is then replaced with the Ramsden and everything brought into sharp focus by moving in or out the draw tube. One must not focus using the microscopes coarse or fine adjustments.

Line up the rulings of the stage micrometer so that a given number corresponds with a particular span of the rulings on the ocular micrometer. Be sure that the rulings are lined up consistently, do not measure from the outside of the line in one place and the inside in another. Use as much of the available rulings as possible for increased accuracy. Write down the rulings on the stage micrometer that are required and the corresponding number from the ocular.

Now divide the distance of the rulings on the ocular by the distance of the rulings on the stage. The dividend is the ocular independent magnification of the objective.

In Practice

A stage micrometer is measured against the Ramsden micrometer eyepiece as described above. Rulings on the stage micrometer are .01mm apart and rulings on the eyepiece micrometer are .1mm apart. It is found that 95 rulings on the stage micrometer correspond exactly to 98 rulings on the eyepiece micrometer.

9.8mm / .95mm = 10.3

The objective then, provides 10.3X magnification.

With a different objective it is found that 15 rulings on the stage micrometer correspond to 65 rulings on the eyepiece micrometer. Once again we work in consistent units of measure.

9.3mm / .22mm = 42.27

The objective then, provides 42.27X magnification.

In Theory

Some users may immediately wonder why it is emphasized that one must focus with a Huygenian ocular, only to replace it with a Ramsden fitted with a micrometer, and manipulate the draw tube for focus. Why shouldn’t one simply use a Huygenian ocular fitted with a micrometer? After all it works for measuring structures.

First consider the construction of a Ramsden ocular. The positive ocular forms a real image below its field lens, outside of the influence of the oculars magnification. A Hugenian ocular, a negative ocular, only forms a real image after light from the objective passes through its field lens. The upshot of which is that a Huygenian ocular will measure an objective as more powerful than it is.

Why then does it mater if one focuses with a negative ocular like the Huygenian but measures with a positive ocular like the Ramsden? The simple explanation is that doing so negates the magnification that results from the eye viewing a virtual image on which a real image of a ruled reticule has been overlaid. Using the Ramsden only will again result in a distortion of the objectives power.

Notes:

∗An eyepiece collar is a small, friction fit, split disc which rides around the outside barrel of an eyepiece and prevents it from seating fully into the microscopes body tube. Such a collar can provide a microscope not equipped with a draw tube with much of the same functionality.

†A Huygenian filar micrometer used with the objectives above measures their powers as 11.37X and 47.8X respectively.

‡In this case the Ramsden eyepiece alone measures the objectives powers as 10.2X and 43.3X. These distortions represent the degree to which the focus was adjusted by manipulation of the draw tube after switching from the Hugenian ocular to the Ramsden.