The Constants of Colour, 
[October, 
466 
care by Aubert, who made an extensive set of observations 
with the aid of coloured discs.* It was found that the ad- 
dition of 1 part of white light to 360 parts of pure coloured 
light produced a change which was perceptible to the eye ; 
smaller amounts failed to bring about this result. It was 
also ascertained that mingling pure coloured light with from 
120 to 180 parts of white light caused it to become invisible, 
the hue being no longer distinguishable from white. Dif- 
ferences in luminosity as small as 1-120 to 1-180 could 
under favourable circumstances be perceived. It hence fol- 
lowed that irregularities in the illumination or distribution 
of pigment over a surface, which were smaller than 1-180 of 
the total amount of light reflected, could no longer be no- 
ticed by the eye. Experiments with red, orange, and blue 
discs were made on the sensitiveness of the eye to changes 
of tone or refrangibility : thus the combination of the blue 
disc with a minute portion of the red disc altered its hue by 
moving it a little toward violet ; on reversing the case, or 
adding a little blue to the red disc, the tone of the latter 
moved in the direction of purple. Similar combinations 
were made with the other discs. Aubert ascertained, in this 
way, that recognisable changes of tone could be produced by 
the addition of quantities of coloured light as small as from 
1-100 to 1-300 of the total amount of light involved. From 
such data he calculated that in a solar speCtrum at least 
1000 distinguishable tones are visible. But we can still 
recognise these tones when the light producing them is sub- 
jected to considerable variation in brightness. Let us limit 
ourselves to 1000 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, differing perceptibly 
from each other. If each of these tones is again varied 
300 times, by the addition of different quantities of white 
light it carries up the number of hues we are able to distin- 
guish as high as 300,000,000. In this calculation no account 
is taken of the purples, or of colours 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 conclusion that 
the human eye under favourable circumstances is able to 
distinguish as many as 400,000,000 different hues ! — From 
Advance Sheets of the New York “ Popidar Science Monthly .” 
* Aubert, Physiologie der Netzhaut. Breslau, 1865. 
