-10 E. L. Nichols — Sensitiveness of the Eye to Colors. 



The variation from these averages, in the case of individual 

 observers, was very marked. Of the fifty-four persons tested, 

 eight (five males and three females), could distinguish the 

 presence of yellow in a mixture of three parts of that pigment 

 in one hundred millions, while two individuals, both of them 

 females, failed to detect it in mixtures containing less than one 

 hundred and ninety parts in one hundred millions. Like 

 differences were met with in the sensitiveness of the eye to 

 other colors, and the relative sensitiveness to different colors 

 was not the same for all observers. 



The lack of delicacy in respect to green was a very general 

 trait. Only three observers were as sensitive to green as to 

 blue, and in the case of but one individual was the power of 

 detecting the former color equal to the sensitiveness to yellow. 

 The thought suggests itself that the failure to detect green may 

 be due to a blunting of the nerves which respond to that color 

 by continual exposure to green foliage. An investigation of 

 the relation between the sensitiveness of the eye to colors 

 mixed with white and the form of the three primary color- 

 curves of the eye would add to our knowledge of this subject. 



The striking discrepancy between these results and those 

 obtained by the method of rotating disks, the eye recognizing 

 with ease and certainty one part of coloring matter in many 

 millions when mechanically mixed with white, and failing to 

 detect one part in a few hundred parts (i. e., 360 parts) when 

 mixed by rotation, shows, in our opinion, that the eye while 

 , watching a revolving disk is in an abnormal condition, and 

 that quantitative results obtained by this favorite method of 

 combining colors are not always comparable with those which 

 we get by the actual mixture of white and colored light, or by 

 the mechanical mixture of pigments. In view of the large 

 number of researches upon Chromatics and Physiological 

 Optics in which the revolving disk has been used, a special 

 study of the condition of the eye during the observation of 

 the disk, and a comparison of the results of this method with 

 those obtained in other ways is greatly to be desired. In this 

 manner alone can the limits of usefulness of this exceeding 

 simple and convenient method be determined. 



The tests of the power of recognition of small differences of 

 shade were undertaken chiefly as a further means of compar- 

 ing the attainments of the sexes in delicacy of color percepti-n. 

 The method was not adapted to the direct determination of 

 the smallest difference of saturation which can be perceived, 

 but our experience with the series of colors already described 

 showed that the neighboring members were quite as closely 

 allied in shade as was compatible with their recognition. 

 Indeed, of fifty-four observers not one succeeded in placing all 



