920 ORIGIN OF VISUAL IMPULSES. [BOOK in. 



the similar instance of shadows cast by objects in the vitreous 

 humour ( 549), the shadow is cast by the rays passing parallel 

 through the vitreous humour; hence the change from shadow to 

 absence of shadow is more marked with the vertical vessels 

 when the movement is sideways and with the horizontal vessels 

 when it is up and down. The fine capillary vessels are seen 

 more easily in this way than by Purkinj^'s method. The same 

 appearances may also be produced by looking through a micro- 

 scope 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 micro- 

 scope 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. 



575. It being admitted that the processes which give rise 

 to visual impulses begin somewhere in the region of the rods 

 and cones, we have to ask the question, How do they begin and 

 what is their nature? We are accustomed to consider light 

 as the undulations of an ether ; a nervous impulse is, so far as 

 we can understand, a molecular change propagated along the 

 substance of the axis cylinder of a nerve fibre ; and, though as 

 we have seen our knowledge of the subject is very limited, still 

 the analogy of a muscular contraction, and of other responses of 

 living substance to a stimulus, lead us to conclude that chemical 

 changes play a part in this molecular change. > By what steps 

 does the undulation of the ether give rise to the material 

 molecular change? In attempting to answer this question 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 more or less 

 direct manner to the molecular vibrations which are the begin- 

 nings of the nervous impulses in the optic nerve. And the rapid- 

 ity with which events must come and go in the retina in order 

 that the eye may be, what it is, an instrument for appreciating 

 rapidly repeated minute changes, lends support to this view. 

 But the present state of our knowledge of physical phenomena 

 does not afford us an adequate explanation of how such a direct 

 transformation can be effected. The recent progress of science 

 tends, it is true, more and more to lay bare the close relations 

 which obtain between optical and electric phenomena, and the 

 latter, as we have so often seen, play an important part in the 

 generation of nervous impulses. Then again many of the phe- 

 nomena of fluorescence seem to supply a bridge between the 

 vibrations of ether, and the vibrations of molecules. But in 

 neither of these directions is it possible, at present at all events, 

 to frame a hypothesis which can be satisfactorily applied to 

 retinal processes. 



