COLOB PHOTOMETBY                                          43

Olass a—^Minimum Sensitivity in              Class B—^Maximum Sensitivity in
 

THE
 

Green
 


 


 


 

THE Green
 


 

Trowbridge -----

W^ade   .........
 

Red

100.0

97.7
 

Green
91.6
97.4
 

Viol.-
blue

95.6
100.0
 

V^hite   ...
Parker   . .
 


 

Red
96.1

85.7
 

V i ol.
Green   blue

100    95.8
100    95.8
 

Hallock  .......
 

100.0
 

90.9
 

96.2
 

Dennett   .
 


 

93.8
 

100    91.5
 

Furness  .......
 

97.9
 

90.8
 

100.0
 

Tufts   . ..
 


 

89.9
 

100    87.8
 

Curtis   ........
 

90.5
 

86.6
 

100.0
 

Bay  .....
 


 

82.9
 

100    93.3
 

Miss M........
 

100.0
 

81.6
 

99.0
Color-blind
 

Group
 


 


 


 

Alsberg   ...
Mr. W.   ...
 


 


 


 

Red
.  30.3
.  35.6
 

Green
88.1
85.5
 


 

Blue
100.0
100.0
 

Mr. 0.....
 


 


 


 

.  35.3
 

100.0
 


 

93.9
 

These results clearly indicate that wide variations in the deter¬
minations of the luminosity of different colors by different individ¬
uals are to be expected from purely physiological causes, and that
in consequence it is necessary to know the peculiarities of color
vision of the individual observer before his results can be of any
value for purposes of comparison. These errors, however, for a
given individual may be regarded as practically constant, and cor¬
rections may be made for them by reference to a fixed standard,
obtained, as Professor Rood suggested, by adopting the mean of a
great number of observations.

Difficulties which have a psychological basis, on the other hand,
give rise to discrepancies which are much more serious, and which at
the same time seem incapable of elimination. They arise from the
well-known fact that color sensations are not simple, but on the con¬
trary highly complex, and that it is practically impossible in ordi¬
nary experience to analyze them into their component elements.
There are involved, in the first place, all the variations in hue which
are found as we pass from the red end of the spectrum to the violet.
It is a matter of common observation that the colors in the lower
part of the spectrum, the reds and oranges, produce an impression of
greater warmth or ^liveliness" than the hues of shorter wave-length,
and this impression the observer is naturally disposed to interpret
in terms of brightness, which constitutes a second and quite distinct
quality of color sensations.

A third element of color sensation is that commonly designated
as saturation, depending upon the proportion of the black-white
sensation which enters into the whole complex. From a physical
point of view these different qualities of color sensation are depend¬
ent, respectively, upon the absolute wave-length, the amplitude of the