1054 VISION. 



the two thresholds is sometimes called the photo-chromatic interval, 

 and differs in extent for different colours, being absent, or almost absent, 

 in the case of red, and greatest for green, light. Another method of 

 comparing sensibility is to find the amount by which a stimulus must 

 be increased or decreased, in order to give a sensation which is perceived 

 as different. The just perceptible difference or threshold of discrimina- 

 tion is a unit which is very commonly used in the measurement of 

 sensibility. In the case of colourless light, this threshold is again only 

 of one kind ; for coloured light the threshold of discrimination may be 

 determined for differences of colour-tone, brightness, and saturation. 



It is also necessary to have some standard by means of which to 

 compare different colours in respect of brightness. This is a matter of 

 considerable difficulty, and the methods which have been devised depend 

 on certain peculiarities of vision ; the subject of hetero-chromatic photo- 

 metry must be postponed till these have been considered. 



A method of comparing different colours in respect of saturation, 

 which is due to Hering, may be mentioned now, as it is capable of being 

 employed without reference to the special theory in connection with 

 which it is used by Hering. Under certain conditions a coloured light 

 may appear colourless, and the white or grey with which a coloured 

 light is matched under these conditions is known as its white " value " 

 ( Valenz). For the present the white value of a given light may be 

 defined as the light with which it would be matched when seen with 

 the dark-adapted eye at a low intensity (see p. 1078). 



The number of colour-tones which can be discriminated in the 

 spectrum is about 160, and others have to be added for the purple 

 series. It has been found that the eye is most sensitive to change of 

 colour-tone at two places, about the lines D and F, and is less sensitive 

 between and on either side of these points. According to Konig, 1 the 

 just perceptible difference at 640 X is 1'28 X; at 590 X, '26 X ; at 540 X, 

 68 X; at 490 X, 16 X; and at 430 X it is 1-06 X. Uhthoff 2 obtained 

 similar results, while others have found the degree of sensitiveness at 

 D as great as or greater than at F. 



The number of degrees of brightness and saturation of a given colour- 

 tone, which can be discriminated, has not been satisfactorily deter- 

 mined. Taking more or less rough estimates, v. Kries 3 has calculated 

 that the total number of different light and colour sensations which we 

 are capable of experiencing would amount to about 500,000. 



Limits of the spectrum. It has been much disputed whether 

 the limitation of the visible spectrum is due to insensitiveness of the 

 retina to infra-red and ultra-violet rays, or whether the invisibility 

 is due to absorption before reaching the retina. As regards the red 

 end, this question seems to have been decided by Aschkinass, 4 who has 

 investigated the absorption of the ocular media by means of a spectro- 

 bolometer, and found that the absorption at the visible limit only 

 amounts to 5 per cent., and that complete absorption does not occur 

 till 1400 X. At the other end the limitation seems to be to a consider- 

 able extent due to absorption ; the limits of different individuals vary 

 here much more than at the red end, and are probably due to differences 

 of pigmentation of the yellow spot and of the lens. The spectrum 



1 Arch.f. Ophth., 1884, Bd. xxx. Abtli. 2, S. 178. 



2 Arch.f. PhysioL, Leipzig, 1889, S. 171. 3 Ibid., 1882, Suppl., S. 58. 



4 Ztschr. f. PsychoL u. PhysioL d. Sinnesorg., Hamburg u. Leipzig, 1896, Bd. xi. S. 44. 



