THE EYE AS AX OPTICAL IXSTKUMENT. 



203 



parts, of which one slides in the other, and blackened, 

 has in front a combination of lenses fixed in the tube 

 hi on the inside, which refract the incident rays of light, 

 and unite them at the back of the instrument into an 

 optical image of the objects which lie in front of the 

 camera. When the photographer first arranges his instru- 

 ment, he receives the image upon a plate of ground glass, 

 g. It is there seen as a small and elaborate picture in 

 its natural colours, more clear and beautiful than the 

 most skilful painter could imitate, though indeed it is 

 upside down. The next step is to substitute for this 



FIG. 27. 



glass a prepared plate upon which the light exerts a per- 

 manent chemical effect, stronger on the more brightly 

 illuminated parts, weaker on those which are darker. 

 These chemical changes having once taken place are per- 

 manent : by their means the image is fixed upon the plate. 

 The natural camera obscura of the eye (seen in a 

 diagrammatic section in Fig. 28) has its blackened 

 chamber globular instead of cubical, and made not of 

 wood, but of a thick, strong, white substance known as the 

 sclerotic coat. It is this which is partly seen between 

 10 



