872 



RECORD OF CURRENT RESEARCHES RELATING TO 



Fig. 8G. 



The achromatized prism for stereoscopic effects can take but 

 three forms. For simj^licity's sake, I will ouly deal with one half of 

 the prism, the other half beius; symmetrical. 



If a right-angled prism (Fig. 85) receive the rays at a perpen- 

 dicular incidence, the whole of the refraction takes place at the 

 inclined second surface, and the distortion is the greatest possible. 



If an isosceles prism (Fig. 86) be used, the deviation is half as 

 great, but still so considerable as to preclude its use. 



But if my form of prism (Figs. 84 and 87) receive the incident 

 Fig. 85. beam on its inclined surface, the angles are more 



nearly equal than in any other form in any other 

 position, and perfect equality may be obtained by 

 modifying the relative distances between objec- 

 tive, prism, and eye-piece. 



These conditions at once indicate the dimen- 

 sions of the prism. When in position it must be 

 large enough to admit all the rays from the 

 objective at the distance seen to give no distor- 

 tion of a known object ; and here we have dis- 

 tinctness and flatness of field also. 



Such is the Isoj)hotal prism, giving its name 

 to the Microscope. In the course of experiments, 

 I have made the prisms of the lightest and 

 densest glass, of the longest and shortest di- 

 verging power, as large as a shilling, and again 

 so small as to slide into a j-inch objective ; and 

 have arrived at what I believe to be the best 

 form for practical use." 

 It need hardly be said that the quality of " Isophotal " is not (as 

 the inventor would seem to imply by the title of the paper) by any 

 means peculiar to this instrument, those of Riddell, Nachet, Stephen- 

 son, and Ahrens, being equally " isophotal." 



We are fortunate in having been for some years the possessor of 

 one of these instruments, which we preserved, not so much on account 

 of the " Isophotal " prism, as for the mechanical curiosity of the 

 arrangement by which " the two equally inclined bodies of the Micro- 

 scope swing on a pivot at their junction to such an extent as to bring 

 one of them vertical, when the instrument becomes a monocular by 

 merely withdrawing the prism." This motion, Mr. Holmes says, 

 " WB-s first devised by me for this Microscope." 



Mr. Wenham subsequently pointed out* the incompleteness of 

 the article in regard to the binocular Microscopes previously made 

 (Nachet's existing form and others being omitted), and also that the 

 " Isophotal prism " is the same as that devised by himself in 

 1860.t 



Mr. Holmes, in reply to Mr. Wenham's first note, wrote : J — 



" Matters of date have nothing to do with the subject, there being 



* ' Engl. Mech.,' xxxi. (1880) pp. 500 ami 5G9. 

 t Sec ' Moil. Mi.T. Jouni.,' 1874, p. 129. 

 t * Engl. Mcch.,' xxxi. (1880) p. 516. 



Fig. 87. 



