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