372 



PHYSIOLOGICAL PHYSICS. [Chap. XXVIIT. 



By their striking on another internal face, as shown 

 in the figure, a second total reflection occurs, the rays 

 passing up into the eye of an observer at E looking 

 straight down. On the table, at a distance of about 

 ten inches from the eye-piece is a sheet of white paper 

 pp, the reflected rays from which pass straight upwards, 

 and reach the eye in lines parallel with the rays from 

 the object. The eye on looking straight down through 

 the small square corner of the prism that is uncovered 

 by the brass case, will see the image of the object on 

 the sheet of white paper. If a pencil be then taken in 

 the hand and held with its point on the paper in the 

 position to draw, after a little practice the image and 

 the point of the pencil can be made to coincide, and 

 thus one is able with the pencil to follow on the 

 paper the image of the object, and so produce an 

 accurate sketch of it. To facilitate the coincidence of 

 pencil and image, a slightly convex lens is placed 

 below the prism to concentrate the light. 



Chevalier's camera liicida is adapted for 

 a microscops in a vertical position. It is represented 



in Fig. 168. The eye- 

 piece of the micro- 

 scope is removed and 

 the camera put on 

 instead, the screen s 

 serving to fix the 

 camera tube to the 

 microscope tube M. At 



Fig. 168.-Chevalier's Camera Lucida. P is a rectangular prism 



by which the rays from 



the object are totally reflected into the tube at right 

 angles. At the end of the tube is a second prism p', 

 which reflects the rays into the observer's eye above. 

 At the same time, rays from a sheet of paper and the 

 point of a pencil, placed on the table ten inches below, 

 reach the eye from the direction p", and thus with the 



