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XXT. — Improved Form of Stephenson's Binocular Prisms. 

 By C. D. Ahkens. 



(Head Uth October, 1885.) 



The arrangement of prisms for the Binocular Microscope devised 

 by Mr. Stephenson has several advantages which make it, in my 

 opinion, the best for microscopic purposes ; for instance, the equal 

 illumination of both fields, the equal size of the two images, and 

 the fact that both images are always in focus together. It is also 

 readily capable of being combined with an erecting arrangement ; a 

 combination which constitutes by far the most satisfactory mode of 

 viewing objects under the Microscope. But the difficulty of keeping 

 the prisms in adjustment is considerable, and is, I think, the reason 

 why they have not come into more general use. 



This difficulty I hope I have overcome by permanently ce- 

 menting together the pah of prisms which divide the rays imme- 

 diately on emergence from the objective. I construct the prisms 

 of ordinary crown glass, and silver the reflecting surfaces by any 

 of the usual methods which give a firm deposit. I then make a 

 very acute glass wedge, of such an angle as to give precisely the 

 proper inclination of the two main prisms to one another, and 

 cement the whole firmly together, as shown in fig. 220. 



Thus the combination can never get out of adjustment, and no 

 cleaning of the reflecting surfaces is required ; moreover, owing to 

 the great angle of incidence and the brilliancy of the silver deposit 

 on a well polished surface, there is very little loss of light. 



Fig. 220. Fig. 221. 



I also devised some years ago an improvement in the upper 

 erecting prism, of which there is no published account. Instead 

 of making the prism of one piece of glass only, and mounting it 

 thus, I cut it in two and separate the two halves by a black glass 

 wedge of suitable angle, cementing the whole together (see fig. 221). 

 Then the central rays coming from the lower binocular prisms 

 fall perpendicularly upon the surfaces of the upper prisms, and, 

 emerging also at right angles to the upper surfaces, continue their 

 course to the eye-pieces without sensible deviation or distortion. 



