512 VISUAL SENSATIONS. [BOOK HI. 



generation of visual impulses; 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 impulses begin in 

 the region of rods and cones is shewn by the phenomena of the 

 blind spot and of Purkinje's figures respectively. 



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 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 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. 492). 

 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 indications of the 

 large branches of the retinal vessels as they diverge from the 

 entrance of the nerve can frequently be recognised. The existence 

 of the blind spot is also shewn 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. 



Purkinje's Figures. If one enters a dark room with a candle, 

 and while looking at a plain (not parti-coloured) wall, moves the 

 candle up and down, holding it on a level with the eyes by the side 

 of the head, there will appear in the field of vision of the eye of the 

 same side, projected on the wall, an image of the retinal vessels, 

 quite similar to that seen on looking into an eye with the ophthal- 

 moscope. The field of vision is illuminated with a glare, and on 



