652 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



are milled heads acting on a pinion fitted witli right- and left-handed 

 screws working together, by which the body-tubes and a a can be set 

 wider or closer to suit the distance of the observer's eyes. 



Fig. 135. 



The action of the prisms may be understood from the diagram 

 (Fig. 136). c c are the two Wollaston prisms placed close together 

 as figured, so that each may receive one-half of the pencil from the 

 objective ; by two internal reflections in each j)rism the rays are thrown 

 into the right-angled prisms h h, from the long surfaces of which they 

 are again totally reflected to total reflecting surfaces of the truncated 

 equilateral prisms a a, and thence up the tubes. In the diagram a a 

 are shown in vertical section, but in actual use in the Microscope, 

 the reflecting surfaces, relatively to b b, would direct the rays from 



