210 RECENT PROGRESS OF THE THEOKY OF VISION. 



crimination of distances is made. The cones are here 

 packed most closely together, and receive light which has 

 not been impeded by other semi-transparent parts of the 

 retina. We may assume that a single nervous fibril runs 

 from each of these cones through the trunk of the optic 

 nerve to the brain, without touching its neighbours, and 

 there produces its special impression, so that the excita- 

 tion of each individual cone will produce a distinct and 

 separate effect upon the sense. 



The production of optical images in a camera obscura 

 depends on the well-known fact that the rays of light 

 which come off from an illuminated object are so broken or 

 refracted in passing through the lenses of the instrument, 

 that they follow new directions which bring them all to a 

 single point, the/ocus, at the back of the camera. A com- 

 mon burning glass has the same property ; if we allow the 

 rays of the sun to pass through it, and hold a sheet of white 

 paper at the proper distance behind it, we may notice two 

 effects. In the first place (and this is often disregarded) 

 the burning lens, although made of transparent glass, 

 throws a shadow like any opaque body ; and next we see 

 in the middle of this shadow a spot of dazzling brilliance, 

 the image of the sun. The rays which, if the lens had 

 not been there, would have illuminated the whole space 

 occupied by the shadow, are concentrated by the refracting 

 power of the burning glass upon the bright spot in the 

 middle, and so both light and heat are more intense there 

 than where the unrefracted solar rays fall. If, instead of 

 the disc of the sun, we choose a star -or any other point as 

 the source of light, its light will be united into a point at 

 the focus of the lens, and the image of the star will appear 

 as such upon the white paper. If there is another fixed 

 star near the one first chosen, its light will be collected at 

 a second illuminated point on the paper ; and if the star 



