16 THE ANNUAL ADDRESS OP THE VICTORIA INSTITUTE. 



reason to suppose that the effect of Hght, whatever it be, on 

 one of these elements (be it cone or be it rod) gives rise to 

 the sensation of a point ; and that the position of that point 

 in the field of view depends upon the position of the element 

 of the bacillary layer which has been affected by the light 

 coming from the point. Moreover in the nerve-fibres which 

 come from the anterior ends of the rods and cones we appear 

 to have a provision for communicating, through the optic 

 nerve, to the brain, the influence, or an indication of the 

 influence, which light exerts on one of these elements. 



Now I have mentioned one argument for believing that 

 this remarkable bacillary layer is that in which light, which 

 previously merely passed through the eye as it would through 

 an optical instrument, acts in some manner on the organism so 

 as to give rise to stimulation of the nerves which convey to us 

 the sensation of vision. The argument, so far, is a sort of d 

 priori one, but it has been remarkably confirmed by an 

 experiment of H. Miiller's, made by means of Purkinje's 

 figures. 



When in a room which is not quite dark we look with one 

 eye towards a moderately Illuminated wall with uniform 

 surface, and holding a candle to one side of the eye move it 

 up and down, there is seen in the field of view a figure 

 branching like sea weed. This is the shadow of the blood 

 vessels of the retina. That the candle requires to be moved 

 in order to show the figure, is explained by the consideration 

 that the shadow is not black, but only darker than its 

 neighbourhood, and when the light is steady the exhaustion 

 of the eye for that part of the field which lies beside the 

 shadow tends to equalise the apparent illumination of the 

 parts in and out of shadow ; whereas when the candle is 

 moved the shadow falls on a new place which had been in 

 full light and therefore partially exhausted, and the previous 

 exhaustion and the new partial interception of light falling 

 on that place contribute to make the shadow sensible. 



The existence of a shadow shows already that the 

 percipient layer of the retina must lie behind the blood 

 vessels. But we may go a step further. By suitable 

 methods of illumination we may cause two spots on the 

 surface of the eye-ball, whose positions can be determined 

 from the circumstances of the experiment, to be alternately 

 virtually the sources of the light which caste the shadow, 

 and the places in the field of view of the shadows of the 

 same vessel in the two positions of the illuminating source 



