180 RECENT PKOGRESS OF THE THEORY OF VISION. 



better estimated, so it may well be with our more exact know- 

 ledge of the eye. For the great performances of this little 

 organ can never be denied ; and while we might consider our- 

 solves compelled to withdraw our admiration from one point of 

 view, we must again experience it from another. 



Regarded as an optical instrument, the eye is a camera 

 obscura. This apparatus is well known in the form used by 

 photographers (Fig. 27). A box constructed of two parts, of 

 .which one slides in the other, and blackened, has in front a 

 combination of lenses fixed in the tube h i on the inside, which 

 refract the incident rays of light, and unite them at the back 

 FIG. 27. 





of the instrument into an optical image of the objects which lie in 

 front of the camera. When the photographer first arranges his 

 instrument, 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 glass a prepared plate upon which the 

 light exerts a permanent chemical effect, stronger on the more 

 brightly illuminated parts, weaker on those which are darker. 

 These chemical changes having once taken place are permanent : 

 by their means the image is fixed upon the plate. 



