ACCIDENTS BY RAIL AND SEA. 157 



matic seuse has fundamental colors or different kinds of perceptive ele- 

 ments, and that the different degrees of incomplete color-blindness are in 

 the inverse ratio to the dimension of the visual field. If the central field 

 is limited to a circle of ten degrees from the fixed point, all the respective 

 characteristics of color-blindness are usually found in it, sometimes 

 within even a narrower range. A feeble sense of color manifests itself in 

 a much wider central field. All the anomalies that can be discovered in 

 an examination of the visual field might, in consequence of the method 

 employed, be explained by a diminution of excitability as well as of the 

 number of the elements. The intermediary zone of the normal visual field 

 or belt of red-blindness has an especial interest, as it furnishes us with 

 tbe opportunity of seeing with our own eyes as the red-blind sees, and 

 consequently of exactly comprehending his abnormal perception. 



According to the theory, we see only yellow and blue in this belt, and in 

 consequence we admit that the red-blind not only caZ^ yellow and blue their 

 principal colors, but moreover see them exactly as the normal observer does. 

 This hypothesis cannot assuredly be proved, but this is not necessary, as 

 the explanation Helmholtz has given of the designation of one of the 

 principal colors of the red- blind is perfectly satisfactory. This circum- 

 stance, however, has given rise, amongst others, to a doubt about the 

 Toung-Helmholtz theory, and to another theory admitting four principal 

 colors to the normal seuse of colors, yellow being classed amongst them. 

 But this is useless . It must not be forgotten that colorless light as well 

 as colored light are subjective perceptions, and that comparison here per- 

 forms an important part. This fact is sufficiently proved by the phe- 

 nomena of contrasts, accidental colors, etc. White is not a color ; it is 

 merely a general, neutral light, and is therefore produced when one kind 

 of element is not more excited than another, or when all the elements 

 are equally excited. 



But as the theory obliges us to admit that the excitation of the 

 perceptive elements of green and violet may in certain cases, as in 

 the instance of the red-blind, supply the perception of white, and 

 not bluish green, and that in certain cases, as in that of the green-blind, 

 the excitation of the perceptive elements of red and violet does not give 

 purple, but white, it is in no wise contrary to the theory to admit that 

 the excitation of the organ perceiving green gives the perception of 

 yellow in cases where all that remains moreover of the system of colors 

 is the complementary color of yellow, that is to say, blue. The excitation 

 of the perceptive organ of green gives the perception of green only 

 on the retina or on a point of the retina which also contains the organ 

 perceiving red. But this is not the place for further developments of 

 this theory. 



2. — CLASSIPIOATION OF THE DIFFEEENT KINDS OF COLOR-BLINDNESS. 



In the preceding we have indicated, in conformity with the theory, 

 the different forms of a defective sense of colors to which, we think, 



