1252 ORIGIN OF VISUAL IMPULSES. [BOOK m. 



( 736), 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 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. 



772. 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 attempt- 

 ing 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 beginnings of 

 the nervous impulses in the optic nerve. And the rapidity 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 trans- 

 formation 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 phenomena 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. 



On the other hand we may perhaps more naturally turn to a 

 chemical explanation. We are familiar with the fact that rays of 



