200 RECENT PROGRESS OF THE THEORY OF VISION. 



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 peculiarities 

 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 condition 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 central pit of the retina, and most ot 

 the fixed entoptic images), are never noticed at all ; and if we 

 want to observe them we must employ unusual modes of illu- 

 mination 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 sensation ; it must, I 

 think, be rather explained as a phenomenon belonging to our 

 power of attention, and I now only refer to the question in 

 passing, since its full discussion 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 imper- 

 fection to my readers, I answer, as I said at first, that I have 

 not done so in order to depreciate the performances of this 

 wonderful organ or to diminish our admiration of its construc- 

 tion. It was my object to make the reader understand, at the 

 first step of our inquiry, that it is not any mechanical perfection 

 of the organs of our senses which secures for us such wonderfully 

 true and exact impressions of the outer world. The next section 

 of this inquiry will introduce much bolder and more paradoxical 

 conclusions than any I have yet stated. We have now seen 

 that the eye in itself is not by any means so complete an optical 

 instrument as it at first appears : its extraordinary value depends 

 upon the way in which we use it : its perfection is practical, not 



