3 86 



PHYSIOLOGICAL PHYSICS. [Chap. xxix. 



the base of the triangle is a little window, just above 

 a in the figure, projecting vertically upwards, in 



which is a needle point. Di- 

 rectly opposite, in the truncated 

 apex of the triangle, is an opening 

 through which the eye to be 

 observed looks. The person at 

 this opening is directed to look 

 across through the window a, as 

 if to a far-off object. At one of 

 the angles of the triangular box 

 are placed two prisms b and 6', 

 in front of which a candle is 

 Fig. 174. The Pimko- placed, the lisjlit from which is 



scope of Helmholtz. 7, 1 , i "" ' j i i 



thrown by the prisms in the ob- 

 served eye. The observer looks through the opening 

 at o towards the eye to be observed, on which he 

 sees three images, being images of the candle flame. 

 They are reflected images ; the first is large, bright and 

 upright (a, Fig. 1 75), a virtual image, the reflection from 

 the surface of the cornea acting as a 

 convex mirror ; the second image b is 

 larger and erect, but dim. It is the 

 reflection from the anterior surface 

 of the lens, a virtual image also. The 

 third image c is small, inverted, and 

 still dimmer, a real inverted image from 

 the posterior surface of the lens, 

 actino- as a concave mirror. Now 



Fig. 175. Pur- 

 kinje's Images. 



when the person whose eye is being 

 observed looks, not through the window to a distant 

 object, but to the needle point in the window, he 

 brings his accommodation into play, and the second 

 image is seen to become smaller and to approach the 

 first, that is to say, the anterior surface of the lens 

 moves forwards. The discovery of the three images 

 is due to Purkinje. 



