THE SENSE OF VISION. 785 



considerable practical importance, since it renders those affected by it incapable 

 of distinguishing the red and green lights ordinarily used for signals. Such 

 persons are, therefore, unsuitable for employment as pilots, railway engineers, 

 etc., and it is now customary to test the vision of all candidates for employment 

 in such situations. It has been found that no satisfactory results can be 

 reached by requiring persons to name colors which are shown them, and the 

 chromatic sense is now commonly tested by what is known as the " Holmgren 

 method," which consists in requiring the individual examined to select from a 

 pile of worsteds of various colors those shades which seem to him to resemble 

 standard skeins of green and pink. When examined in this way about 4 per 

 cent, of the male and one-quarter of 1 per cent, of the female sex are found to 

 be more or less color-blind. The defect may be inherited, and the relatives 

 of a color-blind person are therefore to be tested with special care. Since 

 females are less liable to be affected than males, it often happens that the 

 daughters of a color-blind person, themselves with normal vision, have sons 

 who inherit their grandfather's infirmity. 



Although in all theories of color vision the different sensations are supposed 

 to depend upon changes produced by the ether vibrations of varying rates 

 acting upon different substances in the retina, yet it should be borne in mind 

 that we have at present no proof of the existence of any such substances. The 

 visual purple or, to adopt Mrs. Franklin's more appropriate term, " the rod 

 pigment" was at one time thought to be such a substance, but for the reasons 

 above given cannot be regarded as essential to vision. 1 



That a centre for color vision, distinct from the visual centre, exists in the 

 cerebral cortex is rendered probable by the occurrence of cases of hemianopsia 

 for colors, and also by the experiments of Heidenhain and Cohn on the influ- 

 ence of the hypnotic trance upon color-blindness. 



Intensity. The second of the above-mentioned qualitative modifications of 

 light is its intensity, which is dependent upon the energy of vibrations of the 

 molecules of the luminiferous ether. The sensation of luminosity is not, how- 

 ever, proportionate to the intensity of the stimulus, but varies in such a way 

 that a given increment of intensity causes a greater difference in sensation with 

 feeble than with strong illuminations. This phenomenon is illustrated by the 

 disappearance of a shadow thrown by a candle in a darkened room on a sheet 

 of white paper when sunlight is allowed to fall on the paper from the opposite 

 direction. In this case the absolute difference in luminosity between the 

 shadowed and unshadowed portions of the paper remains the same, but it 

 becomes imperceptible in consequence of the increased total illumination. 



Although our power of distinguishing absolute differences in luminosity 

 diminishes as the intensity of the illumination increases, yet with regard to 

 relative differences no such dependence exists. On the contrary, it is found 

 within pretty wide limits that, whatever be the intensity of the illumination, 



1 In a recently developed theory by Ebbinghaus (Zeilschrift fur Psychologic und Physiologic 

 der Sinnesorgane, v. 145) a physiological importance in relation to vision is attached to this 

 substance in connection with other substances of a hypothetical character. 

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