SEC. 7. ON THE DEVELOPMENT OF VISUAL IMPULSES. 



573. 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, producing 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 en- 

 trance 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 gradu- 

 ally from the black spot sideways 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 fig- 

 ure drawn vary of course with the distance of the paper from 

 the eye. If this distance be known, the size as well as the 

 position of the area of the retina corresponding to the blind spot 



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