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IV. — Refracting Prism for Binocular Microscopes. 



By F. H, Wenham, V.P.R.M.S. 



On June 13th, 1860, before the Microscopical Society, I described 

 a Binocular Microscope with an achromatic prism of the form of 

 Fig. 1. My communication 

 was published in the ' Trans- 

 actions ' at the time. The 

 only prism made on this plan 

 was the one exhibited on that 

 occasion. 



I remarked in my paper 

 that the drawback to this prism 

 was " the great difficulty at- 

 tendant upon its construc- 

 tion," for it will be seen that 

 the upper or crown-glass prism 

 has four facets, which have to 

 be worked separately with all 

 the angles exactly parallel to 

 each other. Having made 

 this prism myself, I had no 

 desire to construct another of 

 this form, and therefore in 

 order to simplify the arrange- 

 ment I transposed the components as represented by Fig. 2, in 

 which the flint prisms are the uppermost. The angles of this 

 diagram are drawn on the original steel templates. 



Messrs. Smith and Beck fitted about a dozen microscopes with 

 these prisms. The first was for the late Mr. Lutwidge, and the 

 next for Mr. Janson, of Exeter. The original one, worked by my- 

 self, is now in the hands of Mr. Crisp, who has made a collection 

 of the different forms of binoculars. Immediately afterwards the 

 present form of reflecting prism was devised, which entirely super- 

 seded the last. This was described before the Microscopical Society 

 December 12th, 1860. In that paper I state, with reference to the 

 late refracting prism, as follows : — " Having still further advanced 

 the definition, by a modification in the construction of the (refract- 

 ing) prism, the performance was so superior to anything preceding 

 it that several were made for parties who had seen the results, 

 and which instruments proved satisfactory to their owners." 



I find that this was the only allusion made to what I considered 

 at the time an obsolete contrivance, and not having given a par- 

 ticular description, it appears to have been twice reinvented ; and as 

 in a late arrangement of erecting binocular I have only recently de- 



