ACCIDENTS BY RAIL AND SEA. 147 



the power of distinguishing colors in the same manner, and that some 

 exhibit a divergence of such a nature as to excite surprise and amuse- 

 ment. The confusion of green and red is very common. But, in many 

 instances, the difficulty consists merely in discriminating between deli- 

 cate shades, while the principal colors are easily distinguished. How 

 shall we explain all this; how find the relation existing between the 

 normal sense of colors and the abnormal j and where must the limit be 

 drawn ? 



1. — THEORETICAL SKETCH. 



Every one knows that a radiant body creates, throughout the ether 

 which surrounds it, undulations which are propagated in every direction. 

 When these undulations — whether they proceed directly in right lines 

 from the radiant body, the sun for example, or whether they are reflected 

 by some intervening body encountered in their course — come in contact 

 with special organs of sense, they produce certain corresponding changes, 

 which, in their turn, excite certain i^erceptions in our mind. If the un- 

 dulations come in contact with our skin, we experience the sensation of 

 heat ; if they strike the retina, that of light. It is consequently our own 

 brxiu that produces both light and heat, resulting from certain changes 

 that take place in our organs of sense (the retina or the skin), although 

 by these names we designate the external cause, when we say that the 

 rays of light and heat proceed from the incandescent body. Neverthe- 

 less, to thoroughly understand this fact it is necessary on the one hand 

 to distinguish between light and heat in an objective sense, which are 

 virtually the same thing, that is to say, undulations of the ethereal 

 medium : and, on the other hand, between light and heat in a subjective 

 sense, which are sensations of an altogether different kind. We have, 

 in the first place, here to consider our perceptions, and as their most 

 immediate cause is found in a modification or activity of our own organs 

 of sense, whatever be the external cause, it is clear that we must seek 

 the explanation of all luminous phenomena in our special organism — 

 the optic nerve ; comprehending by that term the retina, the optic nerve, 

 and those portions of the brain with which they communicate. 



If all the undulations of the luminous ethereal medium were exactly of 

 the same nature, or, on the contrary, if all the elements of our optic 

 apparatus reacted in the same way on every kind of undulations of the 

 ethereal medium, we could with difficulty imagine sensations of light of 

 a different nature. Every specific activity of our apparatus of the optic 

 nerve would produce a perception of light which would vary in degree, 

 but not in quality. We could have no conception of, according to the 

 cases, a light of different kinds, that is to say, of color. But, on the one 

 hand, the science of physics teaches that the undulations of ether are 

 of various kinds, differing in the rapidity, and consequently in the length 

 of the waves; and, on the other hand, the subjective qualities of light, 

 oi our perceptions of color, fall within the range of our daily experience. 



