ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



661 



answer the conditions ; and I have in consequence succeeded in form- 

 ing such a lens, which answers the purpose in a very high degree. 



" My new ' Periscopic eye-piece ' consists of a triple eye-lens, a 

 double convex field-lens, the latter being situated within the focal 

 distance of the former, and a diaphragm, located in the focus, of the 

 equivalent of both lenses. 



" The field of the new eye-piece is considerably larger and flatter 

 than that of Kellner's, and the image is sharply defined to the extreme 

 edge. 



" As the focus of the eye-piece lies behind the field-lens (the same 

 as in Eamsden's eye-piece), it is particularly suitable for micrometers, 

 especially as the divisions are distinctly, and in correct proportion, 

 visible to the extreme edge, which is notably not the case with Eams- 

 den's eye-piece. 



" A micrometer division, placed in the focus of the eye-piece, shows, 

 moreover, very perspicuously the high degree of the correction of the 

 aberrations, while the image transmitted by an objective can be no 

 reliable test, as the aberrations of the objective, especially the dis- 

 tortion, are easily confounded with those of the eye-piece." 



Nachet's Objective Carrier. — ^Every working microscopist has 

 desired a ready means of varying his objectives without the trouble of 

 unscrewing one objective and screwing on another. This difficulty 

 has been partly met by the use of the " nose-piece " ; but this cannot 

 be made conveniently (at least in the case of the heavily mounted 

 English objectives) to carry more than two powers.* 



Fig. 144. 



Fig. 145. 



The attention of M. Nachet having been long directed to this 

 point, he has recently brought out an improved form of his " porte- 

 objectif" (originally made on a suggestion of Professor Thury) which 

 allows the change of objectives to be readily made without as much 

 raising of the body from the stage as is required in screwing and 

 unscrewing. 



It consists (Fig. 144) of a fixed inner cylinder, whose top screws 

 into the bottom of the body, this being embraced by a movable outer 



* See Carpenter's ' The Microscope, &c.,' 6th ed. (1881) pp. 856-7 (2 figs.). 



