448 ORGANS OF THE SENSES. 



of form and structure observed between the rods and 

 cones, from Schultze's researches, appear to consist in this : 

 that the rods are affected only by differences in the in- 

 tensity of the light, while the cones are affected by differences 

 in its quality, that is, by its color. Thus comparative histol- 

 ogy shows that the nocturnal animals (the bat, the hedgehog, 

 and mole), have no cones. We know that it is impossible to 

 distinguish colors in the dark. The night-birds also, have no 

 cones, but simply rods : these are sufficient to enable them 

 to distinguish the differences, not in quality, but in quantity, 

 of the light. On the other hand, the day-birds, especially 

 those which live on small insects of brilliant colors, possess a 

 proportionate and much larger number of cones than man 

 or the other mammalia. 



The impressions produced upon the retina exhibit certain 

 interesting peculiarities; one of these being that these im- 

 pressions last for a certain time after the luminous object has 

 ceased to act, and if short luminous impressions succeed each 

 other rapidly, they are at length confused in one continuous 

 impression. Every one knows that a live coal passed rapidly 

 before the eyes produces the effect of a ribbon or circle of 

 fire ; because the impression produced, as it passes before one 

 point in the retina, lasts until the end of the next revolution ; 

 and these successive impressions are so joined together as to 

 show us, by a line of fire, the path of the luminous point. 



On the other hand, a very bright object, placed against a 

 dark background, always seems to us larger than it really is ; 

 while a black or dark object, placed against a luminous back- 

 ground, appears smaller. In order to explain this phenom- 

 enon it is supposed that the most luminous parts excite, 

 not only those parts of the retina upon which they are de- 

 picted, but also the adjacent parts, and thus encroach upon 

 the images of parts less strongly illuminated : this phenom- 

 enon is known under the name of irradiation. Thus a white 

 triangle, placed against a dark background, appears larger 

 than it really is ; while its edges cease to be rectilinear, and 

 appear as curved lines; in short, the surfaces of the sides are 

 convex ; a black triangle, against a white background, appears 

 to us smaller than it is, and the surfaces of the sides will be 

 concave. A surface divided into lines of equal breadth, and 

 alternately black and white, will appear to contain more 

 white than black, the white lines seeming broader than the 

 others; this explains why Gothic buildings, blackened by 

 time, and standing out against a brilliant sky, appear to us 



