226 RECENT PKOGEESS OF THE THEORY OF VISION. 



sequent observer tries to repeat on his own eyes these 

 experiments as he finds them described, it is of course 

 easier for him than for the discoverer ; but even now there 

 are many of the phenomena described by Purkinje which 

 have never been seen by anyone else, although it cannot 

 be certainly held that they depended on individual pecu- 

 liarities of this acute observer's eyes. 



The phenomena of which we have spoken, and a number 

 of others also, may be explained by the general rule that 

 it is much easier to recognise any change in the condi- 

 tion of a nerve than a constant and equable impression 

 on it. In accordance with this rule, all peculiarities in 

 the excitation of separate nerve fibres, which are equally 

 present during the whole of life (such as the shadow of 

 the blood-vessels of the eye, the yellow colour of the cen- 

 tral pit of the retina, and most of the fixed entoptic 

 images), are never noticed at all ; and if we want to 

 observe them we must employ unusual modes of illumina- 

 tion and, particularly, constant change of its direction. 



According to our present knowledge of the conditions 

 of nervous excitation, it seems to me to be very unlikely 

 that we have here to do with a simple property of sensa- 

 tion ; it must, I think, be rather explained as a pheno- 

 menon belonging to our power of attention, and I now 

 only refer to the question in passing, since its full discus- 

 sion will come afterwards in its proper connection. 



So much for the physical properties of the Eye. If I 

 am asked why I have spent so much time in explaining 

 its imperfection to my readers, I answer, as I said at first, 

 that I have not done so in order to depreciate the perfor- 

 mances of this wonderful organ or to diminish our admi- 

 ration of its construction. It was my object to make the 

 reader understand, at the first step of our inquiry, that it 



