The wide, shallow groove near the end is where the set screws in the outer tube part (which the eyepiece fits into) clamp. The groove allows the position of the outer eye tube (and therefore eyepiece) to be adjusted with a total range around 4 mm relative to the telelens and color-correcting doublet before clamping it in place with the set screws.
I think the basic question here is, what apparent distance is the virtual image from the eyepieces intended to be projected from? If that is known then I am thinking the eye tube position could be set like this:
- Manually focus a camera on an object at the intended virtual image distance.
- With the head removed from the stand point the telelens at a distant object.
- Set tube length by focusing the distant object with the camera looking through the eyepiece in live view mode.
Another guess is that the distance should be consistent with what is implied by the inward angle of the eye tubes.
Does anyone know of a microscope where there is an adjustment like this and also an official service manual available that covers how to set it?
Finally, in the case of the 410, it is not obvious what the main effect of changing this should be -- spherical aberration due to changing the objective working distance? Or lateral CA due to changing the distance from the color-correcting doublet to the intermediate image? Or perhaps both are significant. If other microscopes do not have a similar adjustment that would point to it being related to the doublet.