SENSE OF VISION SUBJECTIVE PHENOMENA. 775 



especially the case in regard to the simple colors, strong combinations of any 

 two of which, without any color that is complementary 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 ob- 

 tained 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. 1 Some persons whose visual powers are excellent in every other 

 respect, are more or less deficient in the power of discriminating colors. 

 This defect (which is now commonly known as " Daltonism," from the name 

 of the distinguished philosopher who was himself the subject of it) may be 

 so complete, that nothing can be perceived save different degrees of light 

 and shadow ; more commonly, however, it exists only with regard to partic- 

 ular colors, especially such as have a complementary relation to one another, 

 so that persons thus affected are unable (e. <?) to distinguish ripe cherries 

 among the leaves of the tree, save by their form ; whilst in some individuals 

 it does no more than confuse colors that are nearly related, such as green 

 and blue, especially when they are seen by artificial light. 2 



632. The three fundamental colors, as was originally shown by Thomas 

 Young, are red, green, and violet. The most common form of color-blind- 

 ness is that for the red rays, which in some cases is incomplete, so that whilst 

 brilliant reds can be distinguished from other colors, low tones are not rec- 

 ognized from gray or black. Purkinje long ago showed that the periphery 

 of the Retina exhibited defective perception of the different colors, red light 

 appearing gray ; yellow, green ; green, bluish ; but blue and violet unaltered. 

 The ordinary form of color-blindness would therefore seem to be only an 

 extension of the want of perceptivity normal in the periphery of the Retina. 3 

 Niemetschek 4 has shown that the affection may either be congenital or ac- 

 quired, and that whilst the retina and refractive media are usually healthy, 

 and the sharpness of vision is normal, there is more or less disturbance or 

 lesion of those portions of the frontal convolutions which intervene between 

 the orbits. 



633. The impressions made by luminous objects upon the Retina are not 

 precisely confined to the spots upon which their rays impinge, but extend 

 themselves to a greater or less distance around ; which phenomenon has been 

 termed irradiation. Thus if we make a circular white spot upon a black 



1 This subject has been most carefully and elaborately investigated by M. Chevreul, 

 whose recent Treatise on Colors has almost exhausted the inquiry into the mode in 

 which the Visual sense of Man is affected by them. For a general view of the nature 

 and attributes of Light, see Art. Light, by Sir J. Herschel, in the last edition of the 

 Encyclopaedia Metropolitana. 



2 See especially the Memoir of Prof. Seebeck, in Poggendorff's Annalen, Bd. xlii 

 (1837); that of Prof. Wartmann, in Taylor's Scientific Memoirs, vol. iv, p. 156; 

 Mr. Pole's case, recorded in the Proceedings of the Royal Society, 1856-57, vol. viii, 

 p. 172, and the interesting commentary upon it by Sir J. Herschel, in idem, vol. x, 

 p. 72. See also Preyer, Centralblatt, 1869, p. 72. A difference appears to exist be- 

 tween the effects of reflected and transmitted light in cases of color blindness. See 

 Jeaffreson, Lancet, 1872, pp. 601, 670. Clarke, idem, p. 635. 



3 For an interesting case in which the patient was affected with monocular green 

 blindness and could yet see blue, though this color, if it were a mixture of green and 

 violet, ought to have appeared violet to him, see Preyer, Centralblatt, 1872, No. 8 ; 

 see also Lebert, Klin. Monatschr. f. Augenheilk., 1873, p. 867. The observations of 

 Schelske (Grafe's Archiv, Bd. ix, Heft iii, p 41) and Woinow (idem, Bd. xvi, Heft 

 i, p. 200) have shown that the peripheral parts of the retina are, even in healthy per- 

 sons, insensible to red, and perhaps to green, the latter color being perceived as yel- 

 low. See also Rahlmann, Abst. in Centralblatt, 1873, p. 357. 



* See Prag. Vierteljahrs., Bd. c, 1868, p. 224. 



