174 THE IXDrCTIO>'S OF BIOLOGY. 



After gazing at a bright light of a particular colour, we see 

 on turning the eyes to adjacent objects, an image of the 

 complementary colour ; shov\^ing that the retina has, for the 

 moment, lost the power to feel small amounts of those rays 

 which have strongly affected it. Such inabilities disappear 

 in a few seconds or a few minutes, according to circumstances. 

 And here, indeed, we are introduced to a conclusive proof 

 that special repair is ever neutralizing special waste. For 

 the rapidity with which the eyes recover their sensitiveness, 

 varies with the reparative power of the individual. In youth, 

 the visual apparatus is so quickly restored to its state of in- 

 tegrity, that many of these photogenes, as they are called, 

 cannot be perceived. "When sitting on the far side of a room, 

 and gazing out of the window against a light sky, a person 

 who is debilitated by disease or advancing years, perceives, 

 on transferring the gaze to the adjacent wall, a momentary 

 negative image of the window — the sash-bars appearing light 

 and the squares dark ; but a young and healthy person has 

 no such experience. AVith a rich blood and vigorous circu- 

 lation, the repair of the visual nerves after impressions of 

 moderate intensity, is nearly instantaneous. 



Function carried to excess, may produce waste so great, 

 that repair cannot make up for it during the ordinarj" 

 daily periods of rest ; and there may result incapacities of 

 the overtaxed organs, lasting for considerable periods. We 

 know that eyes strained by long- continued minute work, lose 

 their power for months or years : perhaps suffering an injury 

 which they never wholly recover. Brains, too, are often so 

 unduly worked that permanent relaxation fails to restore 

 them to vigour. Even of the motor organs the like holds. 

 The most frequent cause of what is called *^ wasting palsy,*' 

 or atrophy of the muscles, is habitual excess of exertion : the 

 proof being, that the disease occurs most frequently among 

 those engaged in laborious handicrafts, and usually attacks 

 first the muscles that have been most worked. 



There has yet to be noticed another kind of repair ; — that 



