760 SIGHT. 



which the impact of the rays of light on the retina is enabled to generate 

 sensory impulses, and the events, or rather series of events, through which 

 these sensory impulses (for, judging by the analogy of motor nerves, we have 

 no reason to think that they undergo any fundamental changes in passing 

 along the optic nerve), by the agency of the cerebral arrangements, develop 

 into a sensation. Such an analysis, however, is, at present at least, in most 

 particulars, quite beyond our power ; and we must therefore treat of the sen- 

 sations as a whole, distinguishing between the peripheral and central phe- 

 nomena, on the rare occasions when we are able to do so. 



The Origin of Visual Impulses. 



646. Of primary importance to the understanding of the way in which 

 luminous undulations give rise to those nervous changes which pass along 

 the optic nerve as visual impulses, is the fact that the rays of light produce 

 their effect by acting not on the optic nerve itself but on its terminal organs. 

 They pass through the anterior layers of the retina apparently without in- 

 ducing any effect ; it is not till they have reached the region of the rods and 

 cones that they set up the changes concerned in the generation of visual im- 

 pulses ; and the impulses here generated travel back to the layer of fibres 

 in the anterior surface of the retina and thence pass along the optic nerve. 

 That the optic fibres are themselves insensible to light and that visual im- 

 pulses begin in the region of rods and cones is shown by the phenomena of 

 the blind spot and of Purkinje's figures respectively. 



647. 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 sideways over 

 the paper away to the outside of the field of vision, at a certain distance the 

 black point of the quill will disappear from view. On continuing the move- 

 ment still further 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 out- 

 line will be drawn circumscribing an area of the field of vision within which 

 rays of light produce no visual sensation. 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, the size as well as the position of 

 the area of the retina corresponding to the blind spot may be calculated 

 from the diagrammatic eye (p. 745). The position exactly coincides with 

 the entrance of the optic nerve, and the dimensions (about 1.5 mm. diameter) 

 also correspond. While drawing the outline as above directed the indica- 

 tions of the large branches of the retinal vessels as they diverge from the 

 entrance of the nerve can frequently be recognized. The existence of the 

 blind spot is also shown by the fact that an image of light, sufficiently small, 

 thrown upon the optic nerve by means of the ophthalmoscope, gives rise to 

 no sensations. 



The existence of the blind spot proves that the optic fibres themselves are 

 insensible to light; it is only through the agency of the retinal expansion 

 that these can be stimulated by luminous vibrations. 



648. Purkinje's figures. If one enters a dark room with a candle, and 

 while looking at a plain (not parti-colored) wall* moves the candle up and 



