ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



843 



Fig. 163. 



observer keeps his finger on a trigger A acting on the spring lever 

 to which the prism is attached, and at the right time moves the prism 

 on one side by a slight pressure, allowing the image to pass to the 

 camera for a fraction of a second, the 

 prism falling back again into its place. 



By a special contrivance in the body- 

 tube each observer can regulate his focus 

 once for all, so that whenever the image 

 is sharp as seen through the eye-piece it 

 is at the same time exactly focused on the 

 sensitive plate. 



This is a better plan than that of M. 

 Bourmans,* who endeavoured to " photo- 

 graph a fugitive object, observing it at the 

 same time throughout the whole duration 

 of the exposure" by the arrangement 

 shown in fig. 163. A B is a vertical tube 

 having a horizontal side-tube C. The 

 body-tube with objective E moves freely in 

 A B. At a b is placed a plane plate of glass 

 silvered on its lower surface with a very 

 thin layer of silver. This will reflect 

 75 per cent, of the rays to the sensitive 

 plate F placed at the end of the short 

 side-tube C, and at the same time sufficient 

 light will be transmitted through the plate to enable the observer to 

 keep the object under observation through the eye-piece A. At c d 

 is a shutter which is kept closed until the focus is adjusted, when it 

 can be raised by turning the button at c. 



Fuess's Petrological Microscopes. — Herr K. Fuess has con- 

 siderably modified his Petrological Microscope, which now has the 

 form of fig. 165.+ 



The body-tube has two openings, one at h for the insertion of the 

 Bertrand lens / , and the other at N for the analyser. The lens is 

 attached to the draw-tube B, which can be raised and lowered by 

 rack and pinion at T ; the plate g is a stop, which prevents it from 

 being drawn out of the tube unless extra pressure is used, when the 

 plate will spring a little and allow the lens to pass. 



The analyser N is attached to the plate o o, by which it slides in 

 or out of the tube as desired. An analyser S can be applied over the 

 eye-piece, and a quartz plate, &c, inserted in the slit above the 

 objective, which can also be centered. 



The polarizer P has a rack-and-pinion motion B, and the milled 

 head of the fine-adjustment is graduated and reads against an 

 index i. 



To the rotating stage-plate is applied a mechanical stage with 



* Cf. Girard's ' La Chambre noire,' 1870, pp. 58-60 (1 fig.), 

 t Cf. Iiosenbusoh's ' Mikroskopische Physiographic,' 2nd ed.. 1885, i. pp 562-4 

 (2 figs.). 



