SEC. 9. ON THE DEVELOPMENT OF VISUAL IMPULSES. 



770. We have already called attention to the important 

 fact that the changes which give rise to visual impulses begin 

 on the outer side of the retina, that the rays of light pass through 

 the inner layers of the retina without, as far as we know, pro- 

 ducing any effect, and do not begin their work until they reach 

 the region of the rods and cones. It is in this region that 

 the energy of light is transformed into energy of another kind ; 

 and the processes here started travel back to the layer of fibres 

 in the inner surface of the retina and thence pass as visual 

 impulses along the optic nerve. That on the one hand the optic 

 fibres themselves are insensible to light and that on the other 

 hand visual impulses do begin in the region of the rods and cones 

 is shewn by the phenomena of the blind spot and of Purkinje's 

 figures respectively. 



The Blind Spot. There is one part of the retina on which rays 

 of light falling give rise to no sensations ; this is the entrance of 

 the optic nerve, and the corresponding area in the field of vision is 

 called the blind spot. If the visual axis of one eye, the right for 

 instance, the other being closed, be fixed on a black spot in a white 

 sheet of paper, and a small black object, such as the point of a quill 

 pen dipped in ink, be moved gradually from the black spot side- 

 ways over the paper away towards the outside of the field of 

 vision, at a certain distance the black point of the quill will 

 disappear from view. On continuing the movement still farther 

 outward the point will again come into view and continue in 

 sight until it is lost in the periphery of the field of vision. If 

 the pen be used to make a mark on the paper at the moment 

 when it is lost to view and at the moment when it comes into 

 sight again, and if similar marks be made along the other 

 meridians as well as the horizontal, an irregular outline will be 

 drawn circumscribing an area of the field of vision within which 

 rays of light produce no visual sensations. This is the blind spot. 

 The dimensions of the figure drawn vary of course with the 

 distance of the paper from the eye. If this distance be known, 



