ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 



261 



prism E is replaced by a larger one, E', cemented upon the gold film 

 (protecting it also from being rubbed off), and a slight inclination is 

 given to the under surface at D', in order to avoid too great an 

 approximation of the pencil to the foot of the Microscope. 



The image of the paper is tinted yellow by the rays reflected from 

 the surface of the gold, while that of the object is of an emerald 

 green tint, that being the colour given to the rays in passing through 

 gold._ 



Fig. 46 shows th,e camera lucida in place over the eye-piece. 



Abbe's Camera Lucida * — Dr. L. Dippel commends the following 

 as an extremely simple and complete apparatus for drawing on a 

 horizontal sui'face. 



A small glass cube A (Fig. 47) composed of two right-angled 

 prisms cemented together is placed over the eye-piece C, one of the 

 prisms having an hypothenuso surface silvered, leaving, however, a 



Fig. 47. 



circular hole. The cube is so adjusted that the hole exactly coin- 

 cides with the " eye-point " of a Zeiss No. 2 ocular (C). The mirror 

 B is connected with the fastening of A by an arm about 70 mm. from 

 the axis of the Microscope. 



In use, the instrument is fastened to the eye-piece cover by two 

 centering screws, and the mirror so turned that the surface of the 

 table close beside the foot of the Microscope appears to be projected 

 on the circular field of the eye-piece. The whole field of view is now 

 readily seen, and with uniform sharpness, and this is the case also 

 when the higher powers are used, no perceptible loss of light taking 

 place in the vision of the microscopical image. One of the most 

 essential qualities of a good camera lucida is therefore obtained. 



That the camera is attached to a particular eye-piece, and is not, 

 as usual, made adjustable for those of different power, arises from the 

 fact that in the higher Huyghenian eye-pieces the eye-point lies too 

 near tbe eye-lens. 



* Bot. Centralbl., ix. (1882) pp. 242-3 (1 fig.). 



