E L. Nichols — Sensitiveness of the Eye to Colors. 41 



the vials, the colors of which were perceptible to him, in their 



proper places in the series. The two nearest approaches to 



complete accuracy consisted in the correct arrangement of 



ninety bottles out of ninety-two in the one case and of ninety 



out of ninety-three in the other. Although these records 



were made by male observers, the average for the other sex 



was noticeably higher than that of the males. It was found 



that of all the mixtures possessing appreciable color the 



average observer of each sex placed the following proportions 



correctly : 



Table II. 



Average accuracy of male and female observers in detecting the degree of saturation of 

 mixtures of pigments with white. ( Complete accuracy would be indicated by lOO'OO.) 



Males 



Females 



Red lead. 



Chromate of lead. 



Chromic oxide. 



Ultramarine. 



86-86 



87-16 



92-81 



78-13 



90-81 



93-24 



98-28 



82-92 



A comparison of tables I and II shows that the color (green) 

 to which the eye is least sensitive, so far as the ability to 

 detect small amounts of color is concerned, is the one in which 

 the least difficulty is met with in noticing differences of shade. 

 Possibly the circumstance already suggested as the cause of 

 the deliciency in the one respect, i. e., continued exposure and 

 consequent loss of sensitiveness to green, may be looked to as 

 the cause of the increased facility in the other. If the detec- 

 tion of colors of low saturation depends upon the delicacy of 

 the eye and the recognition of differences of shade upon prac- 

 tice, it would account equally well for both peculiarities. 



An examination of some of the mixtures used in the fore- 

 going tests under a half-inch objective magnifying about two 

 hundred diameters, showed that the pigments consisted of well- 

 formed, glistening crystals about -f^ mm in diameter. These 

 crystals were mingled with the magnesium carbonate without 

 imparting any trace, of their own color to the latter. Under 

 the microscope the separation was perfect and the contrast of 

 color a striking one. In the more dilute mixtures it was often 

 necessary to search for some time before a single crystal of the 

 pigment could be found, and the portion placed upon the slide 

 did not contain, in some cases, more than five or six crystals 

 altogether. To the naked eye, nevertheless, the mass appeared 

 perfectly homogeneous, and unmistakably colored. Doubtless 

 the power of a few isolated points of color, too small to be rec- 

 ognized individually by the eye, to impart their own hue to 

 the entire colorless field in which they lie, is due to the persist- 

 ence of the color-impression they produce upon the retina; this 

 impression being fused with the impression of white from the 

 remainder of the field of view by the continual movement of 

 the eye in the process of observation. 



University of Kansas, July, 1884. 



