SENSE OF VISION. 421 



has received a strong impression from a coloured object, the spectrum exhibits 

 the complementary colour;* thus, if the eye be fixed for any length of time 

 upon a bright red spot on a white ground, and be then suddenly turned so as 

 to rest upon the white surface, we see a spectrum of a green colour. The 

 same explanation applies to the curious phenomenon of coloured shadows. 

 It may not unfrequently be observed at sunset, that, when the light of the sun 

 acquires a bright orange colour from the clouds through which it passes, the 

 shadows cast by it have a blue tint. Again, in a room with red curtains, the 

 light which passes through these produces green shadows. In both instances, 

 a strong impression of one colour is made on the general surface of the retina ; 

 and at any particular spots, therefore, at which the light is colourless but very 

 faint, that colour is not perceived, its complement only being visible. The 

 correctness of this explanation is proved by the fact, that, if the shadow be 

 viewed through a tube, in such a manner that the coloured ground is excluded, 

 it seems like an ordinary shadow. It is not unlikely that, as Miiller suggests, 

 the predominant action of one colour on the retina disturbs (as it were) the 

 equilibrium of its condition, and excites in it a tendency to the development 

 of a state, corresponding to that which is produced by the impression of the 

 complementary colour; for the latter is, according to him, perceived even 

 where it does not exist; as when the eye, after receiving a strong impression 

 from a coloured spot, and directed upon a completely dark surface or into a 

 dark cavity, still perceives the spectrum. Upon these properties of the eye 

 are founded the laws of harmonious colouring, which have an obvious analogy 

 with those of musical harmony. All complementary colours have an agreeable 

 effect, when judiciously disposed in combination ; and all bright colours, which 

 are not complementary, have a disagreeable effect, if they are predominant : this 

 is especially the case in regard to the simple colours, strong combinations of 

 any two of which, without any colour that is complementary to either of them, 

 are extremely offensive. Painters who are ignorant of these laws, introduce 

 a large quantity of dull grey into their pictures, in order to diminish the glaring 

 effects, which they would otherwise produce; but this benefit is obtained by 

 a sacrifice of the vividness and force, which may be secured in combination 

 with the richest harmony, by a proper attention to physiological principles. 



553. Some persons, who can perfectly distinguish forms, are deficient, 

 through some original peculiarity in the constitution of the retina, in the 

 power of discriminating colours. This is most commonly seen in regard to the 

 complementary colours, especially red and green ; such persons not being 

 able to perceive cherries amidst the leaves on a tree, except by the difference 

 of their form. Several distinct varieties of this affection may be distinguished, 

 however. These have been classified by Seebeck and Wartmann.t 



554. Amongst other curious phenomena of Vision, is the vanishing of 

 images which fall at the entrance of the optic nerve ; as is shown in the fol- 

 lowing experiment. Let two black spots be made upon a piece of paper, 

 about four or five inches apart ; then let the left eye be closed, and the right 

 eye be strongly fixed upon the left-hand spot. If the paper be then moved 

 backwards and forwards, so as to change its distance from the eye, a point will 

 be found, at which the right-hand spot is no longer visible ; though it is clearly 

 seen, when the paper is brought nearer or removed further. In this position 

 of the eye and object, the rays from the right-hand spot cross to the nasal 



* By tile complementary colour is meant that, which would be required to make white or 

 colourless light, when mixed with the original. As red, blue, and yellow are the primary 

 or elementary colours, red is the complement of green (which is composed of yellow and 

 blue); blue is the complement of orange (red and yellow) ; and yellow of purple (red and blue); 

 and vice versa in all instances. 



f Miiller's Physiology, p. 1213; Taylor's Scientific Memoirs, Vol. iv. p. 156, et seq. 

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