INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 873 



no question of priority in dissimilar inventions. My prism does not 

 claim to be either a copy of, or au improvement on, Mr. Wenliam's 

 prism of 1860, as he appears to suppose, but is substantially an inde- 

 pendent invention, made ten years later (1870) in total ignorance of 

 Mr. Wenbam's prism. . . . And I have only to say, in conclusion, that 

 my prism is as different from Mr. Wenbam's prism as a Huygbenian 

 eye-piece is different from a Ramsden eyo-picce." 



Nachet's Microscope with Rotating Foot. — A Microscope was 

 devised some time ago by M. Nachet to embody a rotatory motion in 

 azimuth around the mirror. This was effected as follows : — 



The base of the stand was a solid disk of metal, in the centre of 

 the face of which the mirror was attached with Nachet's usual articu- 

 lations, permitting free motion in all directions. Near the edge of the 

 base a circular groove was made, into which the foot of the Micro- 

 scope proper — a ring of metal carrying the pillar support, as in the 

 Beckett lamp — was fitted, and made to rotate easily. The centre of 

 this circular foot was made coincident with the optic axis of the 

 Microscope. It is obvious that so long as the object remained in the 

 axis of rotation (which was secured by the Microscope being used in a 

 vertical position), the azimuthal rotation around the mirror was 

 practically perfect, except just where the pillar of the stand intercepted 

 the light on the mirror ; and the varying effects of light due to this 

 motion, when the mirror was placed excentrically, could be observed 

 with facility. 



The stand was intended to provide in the simplest form the equi- 

 valent of a perfectly concentric rotating stage, such as is adapted to 

 M. Nachet's more elaborate stands. We have never understood why 

 this inexpensive form of Student's stand has been withheld from 

 popular appreciation. 



Edmunds's Parabolized Gas Slide and Nachet's Gas Chamber.— 

 Dr. Edmunds claims that everything which can be accomplished by 

 means of the latter apparatus * may also be accomplished by the 

 parabolized gas slide.f 



For the study of such coarser microscopical objects as do not need 

 the black-ground effects of the immersion paraboloid, it is necessary 

 only that a ring of tin-foil, wax, or shellac bo interposed between the 

 margin of the thin cover and the toj) of the slide, oil or grease being 

 still used to seal up the interspace. Thus tlie thin cover and the film 

 of material under observation is lifted away from immersion contact 

 with the top of the central paraboloid, and gaseous reagents act in- 

 stantaneously upon the object. If also a slender ring of cotton or silk 

 bo packed into the bottom of the annular channel, it serves to hold 

 water and keep up the humidity of the object when, from any cause, 

 it is less convenient to pass gaseous reagents over a piece of wet cotton- 

 wool before entering the annulus of the slide. 



Dr. Edmunds also considers that the gas chamber introduces a 

 practical ditliculty, inasmuch as on changing the reagent it takes a 

 long time to sweep the chamber clear of its previous contents, and 



♦ See this Jouruul, ante, i>. 707. t I'^iil-. 1' 583. 



