762 SIGHT. 



sclerotic b on the one hand, and from that part of the retina /? where visual 

 impressions begin, on the other. H. Miiller found that the distance /? v thus 

 calculated corresponded to the distance of the retinal vessels from the layer 

 of rods and cones. Thus Purkinje's figures prove in the first place that the 

 sensory impulses which form the commencement of visual sensations origi- 

 nate in some part of the retina behind the retinal vessels, i. e., somewhere 

 between them and the choroid coat ; and H. Miiller's calculations go far to 

 show that they originate at the most posterior or external part of the retina, 

 viz., the layer of rods and cones. It must be admitted, however, that H. 

 Miiller's results were not sufficiently exact to allow any great stress to be 

 placed on this argument. 



In the second method of experimenting the image always moves in the 

 same direction as the light, as it obviously must do. In the first method, 

 where the light enters through the cornea, the image moves in the same 

 direction as the light when the light is moved from right to left, provided 

 the movement does not extend beyond the middle of the cornea, but in the 

 opposite direction to the light when the latter is moved up and down. In 

 Fig. 175, which represents a horizontal section of an eye, if a be moved to , 

 b will move to /?, the shadow on the retina c to y, and the image d to <5. If, 

 on the other hand, a be supposed to move above the plane of the paper, b 

 will move below, in consequence c will move above, and d will appear to 

 move below, i. e., d will sink as a rises. 



It is desirable in these cases to move the light to and fro, especially 

 in the first method, as the retina soon becomes tired, and the image 

 fades away. Some observers can recognize in the axis of vision a faint 

 shadow corresponding to the edge of the depression of the fovea centralis. 



649. The retinal vessels may also be rendered visible by looking 

 through a small orifice, such as a pin-hole in a card placed close to the eye, 

 at a bright field such as the sky, and moving the orifice very rapidly from 

 side to side or up and down. If the movement be from side to side the ves- 

 sels which run vertically will be seen ; if up and down, the horizontal vessels. 

 The fine capillary vessels are seen more easily in this way than by Purkinje's 

 method. The same appearances may also be produced by looking through 

 a microscope from which the objective has been removed and the eye-piece 

 only left (or in which, at least, there is no object distinctly in focus in the 

 field), and moving the head rapidly from side to side or backward and for- 

 ward. Or the microscope itself may be moved ; a circular movement of the 

 field will then bring both the vertically and horizontally directed vessels into 

 view at the same time. 



650. The photo-chemistry of the retina. In seeking to understand how 7 

 it is that rays of light falling upon the region of the rods and cones can give 

 rise to sensory, visual impulses in the optic nerve, we may adopt one or other 

 of two views. On the one hand, we may suppose that the vibrations of the 

 ether are able, through the means of the retinal apparatus of the rods and 

 cones for example, to give rise in some way or other to molecular vibrations 

 which are the beginning of the nervous impulses in the optic nerve. No 

 satisfactory explanation of how such a change can be brought about has 

 been offered, and indeed the difficulties of such a conception are very great. 

 On the other hand, we may more naturally turn to a chemical explanation. 

 We are familiar with the fact that rays of light are able to bring about the 

 decomposition of very many chemical substances, and we accordingly speak 

 of these substances as being sensitive to light. All the facts dwelt on in this 

 book illustrate the great complexity and corresponding instability of the 

 composition of protoplasm. And we might reasonably suppose that proto- 

 plasm itself would be sensitive to light ; that is to say, that rays of light 



