236 RECENT PROGRESS OF THE THEORY OF VISION. 



This part of our inquiry has shown us that the qualities of 

 these sensations can only be regarded assigns of certain different 

 qualities, which belong sometimes to light itself, sometimes to 

 the bodies it illuminates, but that there is not a single actual 

 quality of the objects seen which precisely corresponds to our 

 sensations of sight. Nay, we have seen that, even regarded as 

 signs of real phenomena in the outer world, they do not possess 

 the one essential requisite of a complete system of signs namely, 

 constancy with anything like completeness; so that all that 

 we can say of our sensation of sight is, that ' under similar 

 conditions, the qualities of this sensation appear in the same 

 way for the same objects.' 



And yet, in spite of all this imperfection, we have also found 

 that by means of so inconstant a system of signs, we are able 

 to accomplish the most important part of our task to recognise 

 the same proper colours wherever they occur ; and, considering 

 the difficulties in the way, it is surprising how well we succeed. 

 Out of this inconstant system of brightness and of colours, 

 varying according to the illumination, varying according to the 

 fatigue of the retina, varying according to the part of it affected, 

 we are able to determine the proper colour of any object, the 

 one constant phenomenon which corresponds to a constant 

 quality of its surface; and this we can do, not after long 

 consideration, but by an instantaneous and involuntary de- 

 cision. 



The inaccuracies and imperfections of the eye as an optical 

 instrument, and those which belong to the image on the retina, 

 now appear insignificant in comparison with the incongruities 

 which we have met with in the field of sensation. One might 

 almost believe that Nature had here contradicted herself on 

 purpose, in order to destroy any dream of a pre-existing har- 

 mony between the outer and the inner world. 



And what progress have we made in our task of explaining 

 Sight 1 It might seem that we are further off than ever ; the 

 riddle only more complicated, and less hope than ever of finding 

 out the answer. The reader may perhaps feel inclined to 

 reproach Science with only knowing how to break up with 



