HARD JVICXE'S SCIENCE- G OS SI P. 



3 r 



stances. Further these substances are such that in the 

 red-green substance, red rays falling on it cause dis- 

 similation, and green assimilation ; whilst in the yellow- 

 blue substance yellow rays cause dissimilation, and 

 blue assimilation ; but all rays alike arouse dissimila- 

 tion in the white-black substance. Consequently, 

 when yellow and blue light falls on the eye, dissimila- 

 tion and assimilation in the yellow-blue substance is in 

 equilibrium ; neither process goes on, and neither the 

 sensation of yellow nor of blue is perceived, but since 

 both rays act on the white-black substance, causing 

 dissimilation, we perceive the sensation of white light. 

 This has been shown above to be actually the case. 

 An orange sensation is aroused by dissimilation of both 

 the red-green and yellow-blue substances, purple by 

 assimilation of the yellow-blue substance, and dissimi- 

 lation of the red-green substance, and other inter- 

 mediate colours by other variations in the dissimilation 

 or assimilation of the substances. 



About the beginning of the present century it was 

 discovered that many persons are born with a 

 deficient perception of colour, being what is called 

 " colour-blind," and it was noticed that this deficiency, 

 though found more or less in one out of every eighteen 

 men in England, was very rare in women, even when 

 belonging to a family hereditarily colour-blind. Mr. 

 Hugo Magnus supposes that our sense of colour has 

 been developed during the last four or five thousand 

 years, and that the savages before that period could 

 only distinguish a dark from a light shade. He 

 bases this theory of the evolution of colour on philo- 

 logical researches, but the evident colour perception 

 found in lower animals tends to contradict it. 



To be what is called "red-blind," is the commonest 

 form of colour blindness. A person so afflicted fails 

 to distinguish between rose-red and bluish-green : he 

 sees little more than two colours in the spectrum, 

 which he calls yellow and blue, the yellow including 

 the red, orange, yellow and green spaces. He 

 classes all these tints as yellows, though they are 

 really greens, because yellow being more luminous 

 than green, excites a green sensation even more 

 than green does itself, hence yellows are more con- 

 spicuous than greens to the colour-blind, but only 

 by virtue of the green they contain. The extreme 

 red, if at all faint, is invisible to a red-blind person. 



Maxwell found that, for a colour-blind person, it 

 was only necessary to choose two colours (instead of 

 three), and to combine them, by the aid of his discs, 

 with the addition of black and white, in order to 

 match any other colour ; whereas, for the normal 

 eye, we have seen that the] combination of three 

 colours, with black and white, was required. By 

 comparing the colour equations obtained from his 

 colour top, as adjusted by a colour-blind person, with 

 the equations obtained when the experiment was 

 conducted by a person of normal vision, Maxwell 

 calculated the exact colour-sensation which was 

 wanting in the colour-blind person. 



On the Young-Helmholtz theory, a colour-blind 

 person lacks one of the primary colour sensations ; 

 thus, if he is red-blind, his colour sensations are 

 composed of the green and blue alone. 



On the Hering theory, a red-blind person lacks the 

 red-green visual substance ; hence all his colour- 

 sensations must be made up of blue and yellow. 

 According to this theory, green-blindness cannot exist 

 apart from red-blindness ; indeed, it has not been 

 satisfactorily proved that this is ever really the case. 



The peripheral portions of the normal retina are 

 red-blind, and these parts form a large and increasing 

 proportion in cases of excessive tobacco-smoking. 



A certain shoemaker, named Harris, is recorded to 

 have had scarcely any perception of colour at all, and 

 must therefore have resembled Mr. Magnus's primeval 

 savages. 



In gas-light, we are all in much the same condition 

 as if we were partially violet colour-blind, and by the 

 light of a sodium flame we can place ourselves in a 

 condition somewhat like that of Harris. The light 

 from this flame contains only yellow rays of a par- 

 ticular wave length ; hence yellow objects look white, 

 and all other colours look black, or a pale yellow, if 

 they happen to reflect a modicum of yellow with their 

 normal colour — in other words, we can only distin- 

 guish light and shade effect. 



In all practical questions of colour, we find it 

 necessary, not only to consider the colour of the body 

 or surface itself, but also the colour of the adjacent 

 medium, since the colour of the body will apparently 

 change with alterations in the colour of such medium. 

 This is known as Contrast. Thus, a red object looks 

 brighter on a green than on a red surface. 



All contrast phenomena are the result of one of two 

 distinct causes, being due either to fatigue of portions 

 of the retina, or to fluctuations or error in our visual 

 judgment. Effects produced by fatigue of part of the 

 retina, or negative after-images, as they are generally 

 called, have been so freely published in certain well- 

 known advertisements, that it is scarcely necessary 

 to mention them. 



After staring steadily at a bright red object and 

 then turning the eyes on to a grey wall, we seem to 

 see the object again, but its colour is green instead of 

 red ; the reason being that the grey light reflected 

 into the eye from the grey wall contains all the 

 constituent rays of the spectrum, but the visual 

 substance or nerves capable of awakening a red 

 sensation having been wearied, the other colour 

 sensations predominate, giving rise to a sensation of 

 bluish-green, corresponding to that part of the retina 

 which has been so wearied. The after image is 

 necessarily of a complementary colour to the object, 

 since whenever a colour is subtracted from white 

 light the remaining colour is complementary to it. 



These experiments can be variously modified. By 

 looking at a green object on a sheet of yellow paper 

 and then removing the object, jwe see its after image 



