44 THE USE OF THE MICROSCOPE 



Summary. — The twin-objective binocular has erect 

 images and marked stereoscopic effect, both of which are 

 useful for dissection. Little light is lost in passage through 

 the prisms, since they are made of a specially transparent 

 glass, and the four reflections are total and perfect. Owing 

 to the convergence of the tubes, the focus on a plane 

 object is only perfect at the central front-to-back line 

 of the field, where the axes of the twin objectives should 

 meet. But this lateral difference of focus may sometimes 

 be included, with low magnifications, in the normal range 

 of accommodation of the eyes. Perfect coincidence of the 

 two fields of vision requires: centering of the axes of the 

 matched objectives on the center of the field, when in focus; 

 mutual adjustment of objectives and prism sets; equal 

 optical tube length for both tubes; and matched parfocal 

 eyepieces set at the correct angle. The easy stereoscopic 

 coincidence of the two fields requires that the distance 

 between the eyepoints of the eyepieces should be made to 

 correspond to the interpupillary distance of the particular 

 observer. 



Practical Points 



1. If there is astigmatism in the observer's eyes, it should be 

 corrected by appropriate spectacles, or eyepiece caps. 



2. The distance between eyepoints corresponding to one's 

 interpupillary distance can be obtained with the eyepieces most 

 in use, with the eyes close to the eyelenses. It may be measured 

 in millimeters at each time of use, or the eyepieces may be 

 fastened in that position once for all. 



3. Both tubes should be equally illuminated if transmitted 

 light is used. 



4. Opaque illumination should be used when possible, since 

 it rests the eyes more than transmitted light. 



5. In this instrument the illumination is usually decreased with 

 a higher power; and a brighter light is therefore required than 

 for the unaided eye, or for the single corrected magnifying lens. 



6. The instrument should be tested at regular intervals, the 

 surfaces of prisms and objectives being examined with a lens. 



7. A make of binocular is to be preferred in which the prisms 

 and objectives can be readily cleaned. 



