254 OF SENSATION, AND THE ORGANS OF THE SENSES. 



sion of some particular kind, it seems less susceptible of feebler impressions 

 of the same kind. Thus, if we look at any Brightly luminous object, and then 

 turn our eyes on a sheet of white paper, we shall perceive a dark spot upon 

 it ; the portion of the retina which had been affected by the bright image not 

 being able to receive an impression from the fainter rays reflected by the 

 paper. The dark spectrum does not at once disappear, but assumes different 

 colours in succession, these being expressions of the states through which 

 the retina passes, in its transition to the natural condition. If the eye 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 expla- 

 nation applies to the curious phenomenon of coloured shadows. It may not un- 

 frequently 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 impres- 

 sion 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 pre- 

 dominant action of one colour on the retina disturbs (as it were) the equili- 

 brium 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 com- 

 plementary 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 agree- 

 able effect when judiciously disposed in combination ; and all bright colours 

 which are not complementary have a disagreeable effect if they are predomi- 

 nant : this is especially the case in regard to the simple colours, strong com- 

 binations of any two of which, without any colour that is complimentary to 

 either of them, are extremely offensive. Painters, who are ignorant of these 

 laws, introduce a large quantity of dull gray 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. 



346. 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 

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

 however ; and these have been classified by Leebeck.t 



* By the complementary colour is meant that which would be required to make white 

 or colour ess light, when mixed with the original. As red, blue and yellow are the pri- 

 mary or elementary colours, red is the complement of green (which is composed of yel- 

 low 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 Mailer's Physiology, p. 1213. 



