CHAPTER XXIII. 



THE EYE AND THE PHYSIOLOGY OF VISION. 



That the eye is the seat of vision is a bit of knowledge 

 as old possibly as humanity, but it was not until 1602, when 

 Kepler compared the human eye with the camera obscura, 

 that any considerable attempt was made in the explanation 

 of its function. Some time previous Porta had invented the 

 camera obscura, and so it was but a natural step for Kepler 

 to make this the basis for his explanation of the eye. Kep- 

 ler's explanation was practically nothing more than merely 

 the statement of such an analogy and did not deal with 

 specific details. A few years later, 1609, the Jesuit 

 priest Scheiner noted the inversion of the image on the ret- 

 ina, and made the further important discovery that the 

 pupil contracted or expanded with the varying accommoda- 

 tion of the eye. In 1695 the optician Huyghens constructed 

 an artificial eye and demonstrated on the same the action 

 of spectacles. During the next century nothing seemed to 

 be added to the knowledge of the eye. In 1801 Young 

 (the founder of the present Young-Helmholtz Theory of 

 Vision) noted and explained astigmatism. L,ong-sight and 

 short-sight had been previously observed, and Huyghens 

 had actually explained the action of spectacles in remedy- 

 ing these defects. 



Up to the beginning of this century, and even later, 

 nothing was known of the manner in which the eye was 

 accommodated for far and near. The necessity for such a 

 power of accommodation had been noted by Kepler, and 

 Descartes had actually suggested the thought that the 

 power of accommodation resulted from the ability of the 

 lens to change its form, but it was not until 1801 that 

 Young demonstrated this change in experiments on his own 

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