MKMOIB VI.] HYPOTHESES OF VISION. 109 



conceive of the comparatively slow swaying of groups 

 of molecules under the influence of normal pulsations in 

 the air, but not of the dance of atoms disturbed by trans- 

 verse vibrations in the ether. If, therefore, there were 

 no arguments of an anatomical kind to be presented 

 against the admission of this hypothesis, we should be 

 compelled to turn aside from it because of the inade- 

 quacy of our knowledge in tracing its conditions to their 

 applications. 



This, therefore, is the conclusion at which we finally 

 arrive that vision depends on chemical changes, especial- 

 ly of oxidation, in the retina, and that they approach in 

 their nature those that we speak of as photographic. 

 There is no difficulty in understanding how such changes 

 may give rise to an influence transmitted along the optic 

 nerve to the brain, when we reflect that the oxidation 

 of a few particles of zinc may accomplish specific me- 

 chanical results through many miles of intervening tele- 

 graphic wire, producing mechanical motions as in the 

 telegraph of Morse, or chemical changes as in that of 

 Bain. 



We have remarked that a critical study of the func- 

 tion of vision cannot fail to lead to interesting results 

 respecting the nervous system generally. Guided by 

 that remark, we may perhaps profitably consider further 

 the vestiges of visual impressions, and the physical con- 

 ditions under which they disturb us or spontaneously 

 obtrude themselves on our attention. 



The perception of external objects depends on the 

 rays of light entering the eye, and converging so as to 

 produce images, which make an impression on the retina, 

 and through the optic nerve are delivered to the brain. 

 The direction of these influences, so far as the observer 

 is concerned, is from without to within, from the object 

 to the brain. 



