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Transactions of the 



rays to cross; and being very close to the double-image ones we 

 do not get all the rays of light, only sufficient to be Hluminated, 

 which gives it a very good stereoscopic effect ; and these flint prisms 



bring the centre rays about the 

 same distance apart as they were 

 before they crossed. 



In this arrangement you re- 

 quire a special pair of bodies, both 

 being at an angle. This is the 

 best plan, because you do not lose 

 so much light, being closer down 

 than the binocular eye-piece I have 

 made, for I was asked by several 

 gentlemen if I could not make a 

 binocular eye-piece for high powers, 

 as they did not wish to have any 

 more alterations made on their 

 present form of microscopes. This 

 I have succeeded in doing. I make 

 the prisms go down the upright 

 tube about 5 inches, and the rays 

 cross in this tube as represented in 

 the drawing, and about 4| inches 

 above the upright tube. This is a 

 severe test for it, the rays travelling 

 so far ; loss of light we can make 

 up for by having a stronger hght 

 or a larger mirror. 



This arrangement may not be 

 quite so good as Mr. Wenham's for 

 low powers, but for Mffh powers I 

 cannot conceive of anything finer. 

 This system also gives one of 

 the most perfect micro-polariscopes 

 that we can have, for the double- 

 image prism takes the place of the 

 Analyser; and the prisms being 

 worked so very true compared with 

 what the Nicol's prisms in general 

 are, we get the lull field, but we 

 can use only one body at the time 

 unless there is a Nicol's prism in 



EXPLANATION OF THE FIGURE. 



a, a. Double-image prism of calc-spar, lined with glass b. c, Double flint 

 prism for achromatism of the light, d, The ray of light from the object-glass. 

 Rack-work and milled head are shown. 



