B. O. Peirce—Sensitiveness of the Eye to Color. 299 
Art. XXXIT—On the Sensitiveness of the Eye to Slight Differ- 
ences of Color; by BENJAMIN OsGoop PEIRCE, JR. 
AUBERT has shown* by his experiments with revolving 
’ discs that the eye is able to detect the change produced by the 
addition of 1 part ofswhite light to 360 parts of colored light, 
and that perceptible changes in hue can be brought about by 
adding to light of any color less than 1 per cent of light of a 
ifferent color. He infers from this that a normal eye could 
distinguish at least one thousand different hues in the solar 
Spectrum. 
At the suggestion of Professor Wolcott Gibbs, I have made 
a few experiments to test the sensitiveness of the eyes of differ- 
ent people to slight changes of wave-length in different parts of 
the spectrum. 
For this purpose, a long, thin sheet of vulcanite was inserted 
lengthwise into the collimator of a large spectroscope so as to 
divide the tube into an upper and a lower half. The lower 
part of the tube received light from a fixed slit, the upper part 
fom a movable slit of the same width, which could be set 
exactly over the other, or displaced to the right or to the left. 
The amount of displacement was determined by means of a 
steel scale fastened to the upper slit and moving with it past a 
fixed zero point. The light from the collimator fell upon a 
Rutherfurd diffraction-grating of about 17,000 lines to the 
inch, and the resulting spectra were then thrown, one above 
the other, into the observing telescope. A blackened, metallic 
_ diaphragm, out of which two narrow slits had been cut in the 
Same vertical line, was placed in the eye-piece of the telescope, 
So that when the two collimator slits were even the observer 
Saw merely two narrow strips (one over the other) of the same 
colored light on a black field. When the movable collimator 
slit was displaced, the color of the observer’s lower strip was 
changed without changing its position in the field, and the object 
of the experiments was to see how small a displacement could 
be infallibly detected and named in direction by the observer. 
The width of the collimator slit was about 25°, and the 
slit in the eye-piece diaphragm was nearly of the gd were SIZE 
of the collimator slits as seen through the telescope. 
