48 



THE USE OF THE MICROSCOPE 



distance. But, according to Gullstrand, the normal eye 

 looking through a microscope, is best accommodated for 

 distant vision with the muscles relaxed. Parallel tubes 

 appear to have the following advantages. The point of 

 convergence for the eyes is not altered on altering the 

 distance between the two eyepieces. The nature of the 



Fig. 15. — Another arrangement of prisms for the monobjective binocular. 

 Interocular distance is altered by rotating the two upper prisms. Such adjust- 

 ment does not affect the optical tube length. Note that there are four reflections 

 on each side. This arrangement is doubtless used in the new binocular micro- 

 scope of Zeiss. 



vision, whether feebly or strongly stereoscopic, is not 

 altered by slight changes in the distance of the eye from 

 the eyepiece. The distance between the eyepoints remains 

 the same when the eyepieces are changed for other par- 

 focal ones with longer or shorter collars. Stereoscopic vis- 

 ion is possible by halving one or both of the eyepiece 

 circles, either by special diaphragms, or by sufficiently 

 lessening the distance between the eyepieces. Alteration 

 of the distance between the eyepieces can be efTected, in the 



