mVERTEBRATA, OEYPTOGAMIA, MICROSCOPYj ETC. 871 



[Here follows a description with diagrams of various binocular 

 Microscopes, viz. : — Eiddell's (1851), Nachet's (1852), Wenham's first 

 (1853), Holmes's first (1858), Wenham's second (1860), Holmes's 

 second (1869), Stephenson's (1870).] 



" Here, then, we have six different systems employing prisms 

 having from four to twelve surfaces to absorb light and impair defini- 

 tion. Division and angulation of the objective [Holmes's second 

 plan J disposes of all these sources of error, and gives directness and 

 value to an observation ; but when that plan is considered inadmis- 

 sible, the solution of the problem must rather be sought in the direc- 

 tion of a refracting arrangoaent of few surfaces, as less light is lost 

 and less error introduced thus than by any number of reflecting 

 prisms. 



These considerations led to my third binocular. Fig. 84, wherein 

 one achromatic prism of three surfaces* of the form and in the position 

 shown, divides the light from the objective and bends it into itself, 

 until both its halves cross each other and diverge from opi)osite sides 

 into two eye-pieces. I call this the Isopliotal prism, and it gives the 

 most correct stereoscopic effects to all objects viewed, without im- 

 pairing illumination or definition. 



The light lost by transmission is about 5 per cent., and the gain 

 by observing with two eyes is about 10 per cent. ; therefore, a more 

 brilliant view is obtained in this manner than by a monocular, using 

 the same glass and illuminator. 



The two equally inclined bodies of the Microscope swing on a 

 pivot, at their junction, to such extent as to bring one of them vertical, 

 when the instrument becomes a monocular by merely withdrawing 

 the prism. This motion was ^irs/ devised by me for this Microscojie. 



In applying any refracting prism to a beam of light carrying an 

 image, it is necessary to place the prism in such a position that it 

 shall refract in its least degree — that is, cause the minimum deviation 

 of the beam acted on ; in all other positions the image of a circular 

 spot of light will be elongated to an elliptical form. 



In adapting a pair of prisms to bend the image pencils from the 

 halves of an objective across each other into opposite eyes (Fig. 81), 

 the violence is greater than if they were to be bent only into adjacent 

 eyes. It therefore becomes important to construct the prisms of such 

 a form as to give the least refraction possible for a given angle of 

 glass ; otherwise, the resulting images being elongated in one direc- 

 tion, a distorted view woiild be produced. 



This error is entirely eliminated by attention to two consider- 

 ations. Firstly, the prism must receive its beam at an aiignhir and not 

 at a perpendicular incidence ; secondly, the distance of the prism from 

 the objective in relation to the cyc-piecc must bo such as to make the 

 angles of incidence and emergence equal to each other — no other 

 form or position being admissible without distortion. I claim to bo 

 the first to have recorded this action in connection with this subject, 

 and to have based the construction of an instrument on the deduction. 



* This ahoiild of course bo /"»;• surfaces, which is equivalent to cijlit iu all, 

 the same us th« ytcpheiiaou binocular when the rellccting plute is used. 



