CONSTRUCTION OF THE MICROSCOPE. 



85 



ments were long since devised by Mr. Wenham, 1 with a ^iew to obtain 

 a greater equality in the amount of light-rays forming the two pictures; 

 and he has lately carried one of these into practical effect, with the advan- 

 tage that the compound prism of which it consists, has so nearly the same 

 shape and size as his ordinary stereoscopic prism, as to be capable of being 

 mounted in precisely the same manner, so that the one may be readily ex- 

 changed for the other. The axial ray a, preceding upwards from the 

 objective, enters the prism A B D E r (Fig. 58) at right angles to its lower 

 face, and passes on to c, where it meets the inclined face A B, at which 

 this prism is nearly in contact with the oblique face of the right-angled 

 prism ABC. By internal reflection from the former, and external reflec- 

 tion from the latter, 



about half the beam I is EIB> 5a 



reflected within the first 

 prism in the direction c b, 

 while the other half pro- 

 ceeds straight onwards 

 through the second prism, 

 in the direction c a', so as 

 to pass into the principal 

 body. The reflected half, 

 meeting at d the oblique 

 (silvered) surface D E, of 

 the first prism, is again 

 reflected in the direction 

 d V ; and passing out of 

 that prism perpendicular- I 



ly to its Surface A F, prO-PowellandLea- 



ceeds towards the second- l ^ c l^E^t 

 ary body. The two prisms cular m ^ ange ~ 

 must not be in absolute 



contact along the plane A B, since, if they were, Newton's rings would be 

 formed; and much nicety is required in their adjustment, so that the two 

 reflections may be combined without any blurring of the image in the 

 secondary body. Being (by Mr. Wenham's kindness) the possessor of 

 a prism thus adjusted by himself, the Author can bear testimony to the 

 -excellence of its performance; and he feels sure that for the prolonged 

 observation, under high powers, of objects not requiring the extreme of 

 perfection in definition such, for example, as the study of the Oyclosis 

 in Plants, great advantage is gained from the conjoint use of both eyes 

 by one of the above arrangements. 



1 " Transactions of the Microsc. Soc." N.S., Vol. xiv. (1866), p. 105. 



