404 



THE SENSORIAL FUNCTIONS. 



of the first, consists in admitting the rays through 

 a small aperture into a cavity, on the opposite and 

 internal side of which the retina is expanded, 

 forming a concave, instead of a convex hemisphe- 

 rical surface. The mode in which this arrange- 

 ment is calculated to answer the intended purpose 

 will be easily understood by conceiving a chamber 

 (as represented in Fig. 407), into which no light is 



40/ 



allowed to enter, except what is admitted through a 

 small hole in a shutter, so as to fall on the opposite 

 side of the room. It is evident that each ray will, 

 in that case, illuminate a different part of the wall ; 

 and that the whole external scene will be there 

 faithfully represented ; for the several illuminated 

 points, which constitute these images, preserve 

 among themselves the same relative situation which 

 the objects they represent do in nature ; although 

 with reference to the actual objects they have an 

 inverted position. This inversion of the image is 

 a necessary consequence of the crossing of all the 

 rays at the same point; namely, the small aper- 

 ture in the shutter, through which they are ad- 

 mitted. 



