102 HYPOTHESES OF VISION. [MEMOIR VI. 



I made a series of experiments on these rays, and 

 found that they passed through the different media of 

 the eye, examined separately, and* what is more to the 

 point, through them all collectively, with but little loss. 

 There was no difficulty in obtaining a dark stain on pa- 

 per made sensitive with chloride of silver, and placed at 

 the back of the eye of an ox, from which the sclerotic 

 and pigment had been suitably removed. In a gen- 

 eral manner the media of the eye act like water on the 

 transmissibility of these rays. 



Admitting from these experiments that invisible as 

 well as visible rays reach the retina, we may next con- 

 sider the nature of the impression made upon it, and are 

 thus brought directly to an investigation of the act of 

 vision. 



There are three hypotheses to be considered : 



1. That rays falling on the retina or black pigment 

 impart to those structures a rise of temperature. This 

 may be termed the calorific hypothesis. 



2. That rays falling on the retina occasion a chemical 

 change or metamorphosis in its structure, implying the 

 occurrence of waste in it, and therefore the necessity of 

 repair. This may be termed the chemical hypothesis. 



3. That rays falling on the retina throw its parts into 

 a vibratory movement, not necessarily attended by any 

 metamorphosis of tissue, as waves of sound occasion con- 

 sentaneous pulsations in the auditory apparatus of the 

 ear. This may be termed the mechanical hypothesis. 



FIRST, of the calorific hypothesis of vision. Compara- 

 tive anatomy offers certain facts which lend plausibility 

 to this hypothesis. Some of the most remarkable of 

 these relate to the construction of the eye in lower ani- 

 mals. The ocelli, which consist of dark-colored or black 

 spots, or black cup-shaped membranes containing w r ithiri 

 them the rudiment of an optic nerve, are the beginning 



