the Sense of Sight. 171 



the rapid changes in our sensation of sight are, I believe, due to 

 slight shiftings, in obedience to the changing play of the light, 

 of the granules between the highly sensitive rods of the retina. 

 The distances through which the granules have to move 

 individually can be shown to be infinitesimal. This fact is of 

 importance, because one of the difficulties against any such 

 hypothesis has been stated to be the great distance the 

 granules would have to travel from the pigment-cells all the 

 way up between the rods, and the consequent slowness of the 

 reaction, which we know from experience to be practically 

 instantaneous. Closer inspection shows that this difficulty 

 does not exist. No eye is fit for vision in which the granules 

 are contracted into the body of the pigment-cells, as any one 

 may prove for himself who suddenly opens the shutters of a 

 really dark room in which he has passed the night. The 

 eye, indeed, has to be prepared for, or, in other words, to 

 " get accustomed to," the light. The general sensation of 

 light must precede tiie sensation of any distinct image, and 

 this general sensation is brought about by the pressing 

 forward of the granules between the sensitive rods. Before 

 we have any distinct vision, therefore, the granules are 

 already in position, actually causing the general sensation, 

 Not until this is the case are we conscious of any definite 

 images. 



Leaving colour-sensation for the moment out of the 

 question, these definite images are really only variations in 

 the intensity of the light. These will be felt in the following 

 way. Where a bright light falls, more granules push forward 

 from the back ; where a shadow falls, the push from behind 

 is relieved. The movements of the individual granules in 

 order to effect these changes of pressure need only be infini- 

 tesimal. In a crowded gangway it is often difficult to see, by 

 any actual movement, who is pushing and who is not. We 

 all, however, feel instantaneously both when the pressure is 

 put on and when it is taken off. No one who has watched 

 the movements of minute organisms under the microscope can 

 doubt that they would supply us with far greater rapidity 

 than that required by this theory, for the instantaneous appli- 

 cation or withdrawal of pressure, the distances to be traversed 

 being, as already stated, infinitesimal. 



We may, then, describe our ordinary vision (apart from 

 colour) as due to constantly varying degrees of relief from, or 

 increase in intensity in, the general sensation of light 

 which, during all vision, floods the eye, this general sensa- 

 tion being caused by the granules pressing forward between 



rdinj 



