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

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