GENERATION OF ANIMALS, V. i. 



fluid which they contain, for the Hght is weak during 

 the night, and, in addition to that, fluid generally is 

 not easily set in movement at night. To obtain the 

 best results, it must avoid both (a) not being set in 

 movement at all and also (6) being set in movement 

 too much in respect of its transparency, because the 

 stronger movement ousts the weaker." That is why 

 people who have been looking at strong, brilliant 

 colours, or who go out of the sunlight into the dark, 

 cannot see : the movement which is already present 

 in their eyes is so strong that it precludes the move- 

 ment which comes from without. And in general, 

 neither strong sight nor weak sight can see bright 

 things because the action undergone by the fluid in 

 the eye is unduly intense — i.e., the fluid is set in 

 movement unduly. This is borne out by the ailments 

 besetting either kind of sight. Cataract tends to 

 attack the blue-eyed more than the dark-eyed, night- 

 bUndness ^ as it is called attacks the latter. Cataract 

 is a sort of dryness of the eyes, and that is why it 

 occurs oftener in the ageing, as these parts (the 

 eyes), like the rest of the body, become dry towards 

 old age. Night-blindness is superabundance of fluid, 

 and that is why it tends to attack younger people : 

 their brain is more fluid.'' The best sight of all is 

 that which is midway between a large amount and a 



children are bluish ; and the reason given for blueness at 

 780 b 1 (and 779 b -29) is the small amount of fluid. At 

 779 b 1 1 , however, the reason given for bhieness is weakness 

 (weakness is explained at 780 b 7 as being due to lack of 

 concoction of the fluid) ; and at 780 b 8 undue thinness of 

 fluid is said to " be equivalent " {Trjv avTTjv exet Stivafxiv) to a 

 small amount of fluid. We may deduce, therefore, that a 

 large amount of thin fluid is equivalent to a small amount of 

 fluid ; at any rate, this seems to be the only way of reconciling 

 Aristotle's apparently contradictory statements. 



499 



