648 THE POPULAR SCIENCE MONTHLY. 



found that the addition of one part of white light to 360 parts of 

 pure colored light produced a change which was perceptihle to the 

 eye ; smaller amounts failed to bring about this result. It was also 

 ascertained that mingling pure colored light with from 120 to 180 

 parts of white light caused it to become invisible, the hue being no 

 longer distinguishable from white. Differences in luminosity as small 

 as __i._ to -J could under favorable circumstances be perceived. It 

 hence followed that irregularities in the illumination or distribution 

 of pigment over a surface, which were smaller than ^ of the total 

 amount of light reflected, could no longer be noticed by the eye. 

 Experiments with red, orange, and blue disks were made on the sen- 

 sitiveness of the eye to changes of tone or refrangibility ; thus the 

 combination of the blue disk .with a minute portion of the red disk 

 altered its hue by moving it a little toward violet ; on reversing the 

 case, or adding a little blue to the red disk, the tone of the latter 

 moved in the direction of purple. Similar combinations were made 

 with the other disks. Aubert ascertained, in this way, that recogniz- 

 able changes of tone could be produced by the addition of quanti- 

 ties of colored light as small as from ^ to ^ of the total amount 

 of lio-ht involved. From such data he calculated that in a solar 

 spectrum at least 1,000 distinguishable tones are visible. But we can 

 still recognize these tones when the light producing them is subjected 

 to considerable variation in brightness. Let us limit ourselves to 

 1,000 slight variations, which we can produce by gradually increasing 

 the brightness of our spectrum, till it finally is ten times as luminous 

 as it originally was. This will furnish us with a million tones, differ- 

 ing perceptibly from each other. If each of these tones is again 

 varied 300 times, by the addition of different quantities of white 

 lio-ht, it carries up the number of hues we are able to distinguish as 

 high as 300,000,000. In this calculation no account is taken of the 

 purples, or of colors which are very bright or very faint, or mixed 

 with very much white light. For these it will hardly be extravagant 

 to demand another 100,000,000 ; we reach thus the astonishing con- 

 clusion that the human eye under favorable circumstances is able to 

 distinguish as many as 400,000,000 different hues ! 



