NATURAL PHILOSOPHY. 141 



the results of this theory would lead us further than is necessary for the 

 purpose we have now in view. 



We also find philosophers now-a-days calmly discussing a question which 

 most people considered settled very long ago, namely, whether blue and 

 yellow together really make green. 



It is of no use for the artist to lift up his eyes with astonishment at any one 

 being so insane as to question so generally admitted a statement. In vain 

 does he point to his pictures, in which his greens have been actually so pro- 

 duced. The strict photologist at once puts him down, by informing him that 

 he knows little or nothing of the real state of the case : his (the artist's) col- 

 ors are negative, or hues of more or less complete darkness ; whereas in nature 

 the color question is to be decided by positive colors, or hues in which all the 

 light used is of one kind. The meaning of this will be best understood by 

 an example: When a ray of white light falls on a green leaf, part of the 

 ray is absorbed and part reflected, and the object is therefore only seen with 

 the part that is reflected. That which is absorbed consists of some of each 

 of the color-rays, and the resulting reflected light is nothing more than a 

 mixture of what remains after this partial absorption. The green we see 

 consists of the original white light deprived of a portion of its rays. It is 

 not a pure and absolute green, but only a residual group of colored rays, and 

 thus in so far the green color is negative, or consists of rays not absorbed. 

 It is therefore partial darkness, and not absolute light. If, however, on the 

 other hand, a ray of white light is passed through a transparent medium 

 (e. y., some chemical sal;) which has the property of entirely absorbing all 

 but one or more of the color-rays, and no part of the remainder, then all the 

 light that passes through this medium is of the one color, or a mixture of the 

 several colors that pass; and if such" light is thrown on a white ground, the 

 reflected color will be positive, and not negative, and is far purer as well as 

 brighter than the color obtained in the other way. It has been found by 

 actual experiment that when positive blue, thus obtained, is thrown on 

 positive yellow, the resulting reflected color bears no resemblance to gi-een. 

 Sir John Herschel considers that whether green is a primitive color in 

 other words, whether we really have three or four primitive colors remains 

 yet an open question. 



It was necessary to explain these matters about color before directly 

 referring to the subject of this paper, namely, blindness to certain color-rays. 

 It should also be clearly understood that the persons subject to this peculiar 

 condition of vision have not, necessarily, any mechanical or optical defect in 

 the eye, as an optical instrument, which may be strong or weak, long-sighted 

 or short-sighted, quite independently of it. Color-blindness does not in any 

 way interfere with the ordinary requirements of vision, nor is there the 

 smallest reason to imagine that it can get worse by neglect, or admit of any 

 improvement by education or treatment. 



Assuming that persons of ordinary vision see three simple colors, red, 

 yellow, and blue, and that all the rest of the colors are mixtures of these 

 with each other and with white light, let us try to picture to ourselves what 

 must be the visual condition of a person who is unable to recognize certain 

 rays; and as it appears that there is but one kind of color-blindness known, 

 we will assume that the person is unable to recognize those rays of white 

 light which consist of pure red and nothing else. In other words, let us 

 invest igate the sensations of a person blind so far only as pure red is 

 concerned. 



