ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



493 



end a shelf for supporting the Microscope. Near the upper end is a 

 sliding-piece to which is attached the box or bellows of an ordinary 

 camera. Under the shelf another piece of board is fastened to the 

 first at right angles : this assists in supporting the shelf and also serves 

 as a leg to help keep the apparatus in an upright position. The lens 

 of the camera is removed and a washer of felt is glued to the edge of 

 the collar, so as to make a light-tight connection with the eye-piece of 

 the Microscope. A slit is made in 

 the side of the collar, and through 

 this slit is fitted an elliptic-shaped 

 piece of metal having a round open- 

 ing in one side, the other side being 

 left entire, and also having a piece of 

 the metal projecting on one side of the 

 ellipse to be used as a handle. The 

 elliptic piece is the shutter for ad- 

 mitting or cutting off the light, and is 

 manipulated by the projecting handle. 

 Specimens of the results obtained are 

 given. 



Photomicrographs on G-elatino- 

 bromide Films.* — W. Forgan, in a 

 lecture before the Edinburgh Photo- 

 graphic Society, narrates how he 

 cleared up the doubt as to the suit- 

 ability of collodion or gelatino-bro- 

 mide plates for photographing eclipses. 

 Microscopical examination showed 

 that the grains of silver in the two 

 plates were of practically equal size, 

 viz. about xy^joth of an inch in dia- 

 meter. The method of preparation, 

 however, of a collodion plate has the 



effect of covering only the surface with a film of silver ; whereas, in the 

 other plate, the silver is thoroughly dispersed throughout the whole 

 medium. This fact seems to account for the superior rapidity of the 

 gelatino-bromide. But the more rapid the action of the plate, the 

 coarser was the granulation. In the ordinary plates the silver grains are 

 in a more scattered form, and the granulation, therefore, finer. Hence, 

 the maker's advice to use ordinary plates wherever possible, is based on 

 sound principles. For astronomical photography, especially for nega- 

 tives where delicate measurements afterwards require to be made, a slow 

 ordinary plate is an essential requisite. 



Fig. 103. 



(5) Microscopical Optics and Manipulation. 



Prisms and Plates for showing Dichromatism.f — R. W. Wood 

 describes how to observe the property of dichromatism, i.e. the change 

 of colour of an absorbing medium with increase of thickness. Thus thin 



* Eng. Mech./lxxv. (April 18, 1902) p. 203. 

 t Nature, lxvi! (1902) p. 31. 



