Most stereo microscopes have slightly angled convergent ocular tubes , in order to maintain the natural angle that our eyes take when we view objects at close focus. This degree of convergence is necessary because stereo microscopes simulate the use of our eyes naturally but as though we are using a strong magnifying glass separately with each eye.
Binocular higher power microscopes do not work like this, having both eyes viewing the same image that has been split. Early binocular microscopes were based on a convergent binocular model but when the beamsplitter began to be employed most companies adopted either a Jentsch binocular head or a Seidentopf type, both of which dispense with convergent optics. A person with a slight difficulty maintaining convergence, or as mentioned above having even a mild case of strabismus could find non convergent binoculars difficult and convergent binoculars o.k.
In the early days of the development of split image binocular tubes there was controversy over the best way to make binocular instruments comfortable. Apparently, many early microscopists did not like binocular tubes. Since they all had been trained on monoculars, a certain number in trying out the new technology found them unusable. Spencer, even went to the trouble of offering a head, where one eye of the binocular could be blocked off and even better, where the head could be shifted sideways to create a monocular head. It seems quaint today but in a lab with only one microscope used commonly, it may have been the cat's meow.
Further, for many years Spencer made binocular heads with Jentsch type heads that had conveniently angled eyetubes at the physiological norm of 4 degrees, which maintains a convergence point of 17" at an interpupillary distance of 62mm.
Spencer-AO, continued to mfg. these convergent heads up until the series 20 100 watt microscope, switching to Seidentopf heads subsequently.
If you can find someone with a Spencer or AO binocular made prior to about 1980, either infinity corrected or 160mm, you might find that one of these works better for you but since you are in England, they aren't that common. A check of some available catalogues might reveal whether any of the older U.K. companies did the same. AO and Baker had a working relationship after the war, until AO purchased Reichert in 1962 and even shared optics. Perhaps Baker, previously had similar heads.
The question might come up of why would it have seemed important to have a convertible head in 1920 and yet such difficulties with binocular heads seem uncommon now? The answer lies in the demographics associated with the early microscopist community, who trained on monocular scopes and the demographics of modern microscopists, most of whom trained on binocular scopes. Have you ever tried to show a child a view in the microscope, only to have them say, "I have to use one eye, it makes me cross eyed when I use both eyes" and no matter how you adjust the interpupillary distance they still can't use both eyetubes. Those are the the kids destined to not be microscopists in the era of binocular microscopes , because the microscopes they are trying will not work for their eyes and kids like that get weeded out of microscopy despite the interest level. In 1910, they might still have continued.
A binocular microscope can be critically adjusted to accomodate a unique convergence but it takes a very hands on approach, with the microscope and user present.
It is also one more reason why the first microscopes for children should be stereo microscopes.
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