108 HYPOTHESES OF VISION. [MEMOIR VI. 



so that such may be considered to be the practical 

 effect. 



THIKD, of the mechanical hypothesis of vision. There 

 is a growing belief among those who are cultivating 

 photo-chemistry that the mode of operation of a ray of 

 light in accomplishing chemical changes is by establish- 

 ing vibratory movements among the molecules of the 

 substance affected. As has been affirmed, perhaps fanci- 

 fully, of certain singers, that they could cause a glass 

 goblet to fly to pieces by a proper intonation of their 

 voice, through the attempt of the glass by resonance to 

 execute incompatible vibrations, so it is thought that an 

 incident ray may break asunder a group of molecules by 

 establishing among them discordant agitations. Chem- 

 ical decompositions by radiations become thus connected 

 theoretically with vibratory movements. 



But these are vibrations not necessarily attended by 

 any destruction of tissue. Waves of sound occasion such 

 pulsations in the apparatus of the ear without producing 

 any chemical change in the auditory nerve. 



If we consider the retina as an elastic shell, of which 

 the parts are put into a purely mechanical movement 

 by the pulsations of light, we abandon without explana- 

 tion some of the most interesting portions of the struct- 

 ure of the eye. Of what use is that wonderful net- work 

 of vessels constituting the choroid ? It is a principle in 

 physiology that the supply of blood to a part is propor- 

 tional to its functional activity. The elaborate vascular 

 mechanism in juxtaposition with the retina will bear no 

 other interpretation than that that tissue is the seat of 

 incessant chemical changes. 



Moreover, physical science in its present state is not 

 sufficiently advanced to furnish the means of clearly 

 comprehending such purely mechanical motions exe- 

 cuted by the ultimate particles of things. We may 



