262 RECENT PROGRESS OF THE THEORY OF VISION. 



which we have laid before the reader illustrate them in 

 the clearest manner. 



After this excursion into the world of abstract ideas, 

 we return once more to the subject of colour, and will 

 now examine it as a sensible ' sign ' of certain external 

 qualities, either of light itself or of the objects which 

 reflect it. 



It is essential for a good sign to be constant that is, 

 the same sign must always denote the same object. Now, 

 we have already seen that in this particular our sensations 

 of colour are imperfect ; they are not quite uniform over 

 the entire field of the retina. But the constant move- 

 ment of the eye supplies this imperfection, in the same 

 way as it makes up for the unequal sensitiveness of the 

 different parts of the retina to form. 



We have also seen that when the retina becomes tired, 

 the intensity of the impression produced on it rapidly 

 diminishes, but here again the usual effect of the constant 

 movements of the eye is to equalise the fatigue of the 

 various parts, and hence we rarely see after-images. If 

 they appear at all, it is in the case of brilliant objects like 

 very bright flames, or the sun itself. And, so long as the 

 fatigue of the entire retina is uniform, the relative 

 brightness and colour of the different objects in sight 

 remains almost unchanged, so that the effect of fatigue 

 is gradually to weaken the apparent illumination of the 

 entire field of vision. 



This brings us to consider the differences in the pictures 

 presented by the eye, which depend on different degrees 

 of illumination. Here again we meet with instructive 

 facts. We look at external objects under light of very 

 different intensity, varying from the most dazzling sun- 

 shine to the pale beams of the moon ; and the light of 

 the full moon is 150,000 times less than that of the sun. 



