CHAP. IL] SIGHT. 515 



orifice very rapidly from side to side or up and down. If the 

 movement be from side to side, the vessels which run vertical 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 backwards and forwards. 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. 



The Photochemistry of the Retina. In seeking to understand 

 how 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 satis- 

 factory 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 

 protoplasm itself would- be sensitive to light; that is to say that 

 rays of light falling on even undifferentiated protoplasm might set 

 up a decomposition of that protoplasm and so inaugurate a mole- 

 cular disturbance ; in other words, that light might act as a direct 

 stimulus to protoplasm. As a matter of fact, however, such evi- 

 dence as we at present possess goes to shew that native undifferen- 

 tiated protoplasm is as a rule not sensitive to light (that is, to those 

 particular waves which when they fall on our retina give rise in us 

 to the sensation of light), though in the case of some lowly organ- 

 isms whose protoplasm exhibits very little differentiation and in 

 particular contains no pigment, a sensitiveness to light has been 

 observed. Nor can we be surprised at this indifference of proto- 

 plasm when we reflect that what we may call pure protoplasm is 

 remarkable for its transparency, that is to say the rays of light pass 

 through it with the slightest possible absorption. But in order 

 that light may produce chemical effects, it must be absorbed ; it 

 must be spent in doing the chemical work. Accordingly the first 

 step towards the formation of an organ of vision is the differentia^ 



332 



