276 GRAY, ON A DOUBLY-REFLECTING PRISM. 



pose of illuminating the object by oblique light. A figure 

 and description of this prism will be found in Quekett, 

 3rd edition, pp. 141, 142, and in the 'Mic. Trans./ 1st series, 

 vol. iii, pp. 74 to 81. The pages last cited contain a mathe- 

 matical investigation of the form of the prism by Mr. Shad- 

 bolt, with whose results, so far as he has given them, mine 

 agree ; but it will readily appear that I have made no use of 

 his investigation. 



The next application to the microscope of the doubly-re- 

 flecting prism is by Mr. Wenham. The purpose of Mr. 

 Wenham's prism is distinct from that of M. Nachet. Being 

 placed behind the object-glass, it receives a portion (say half) 

 of the emergent rays, and deflects them into a supplementary 

 body, attached to the principal body at an angle correspond- 

 ing to the angle of deviation of the prism. These deflected 

 rays form, in the supplementary body, an image, which, while 

 symmetrical with that formed by the remaining rays in the 

 principal body, possesses yet such an amoutit and kind of dis- 

 similarity as to afibrd, when the images are viewed simul- 

 taneously by the two eyes, the effect of perfect stereo- 

 scopic vision. It is the interest excited amongst microsco- 

 pists by the wonderful and startling effect of this binocular 

 arrangement (numerous inquiries on the subject of the prism 

 having been addressed to me) that has induced me to enter 

 upon the present investigation. 



The two prisms are identical in principle, and the pre- 

 ceding investigation and formulae apply equally to both. 

 Their difierences (apart from the lens cemented on the upper 

 surface of Nachet^s) are matters of detail, the diflerent pur- 

 poses to which the prisms are applied requiring different 

 values of ^ and d. 



Before illustrating the 

 formulae by their applica- 

 tion to Mr. Wenham's 

 prism, I give a geometrical 

 construction. 



In the indefinite straight 

 line EF take AG=j9. From 

 G draw the perpendicular 

 GB, and from A draw AB, 

 making with AG an angle 

 equal to 4:5° + ^d, and intersecting GB in B. Also from A 

 draw AH, making the angle GAH = t/, and from B draw BC 

 parallel to AH, and equal to AG=jo. From B let fall BH at 

 right angles to AH. Join CH, and produce it to D. ABCD 

 is the figure required. 



