VII
COLOR PHOTOMETRY

Reference has already been made to the very obvious truth that
in an adequate study of visual acuity the accurate determination of
the intensity of illumination is a matter of the highest importance.
So long as only a single color is involved, the variation of the inten¬
sity in fixed ratios in accordance with the law of inverse squares or
by the use of the episkotister is a very simple matter. Or if different
sources of light of the same hue are employed, their relative intensi¬
ties may be determined with the more sensitive forms of photom¬
eters now in general use with an error of less than one per cent.
It is possible, moreover, to make the measurements of intensity
purely physical in character, if desired. The energy of the source
may be determined directly by bolometric methods, or the chemical
effects may be registered with minute shades of difference on the
photographic plate. Quite recently the selenium cell has been pro¬
posed as affording more accurate and refined measurements of light
intensity than any method hitherto used.

When, however, differences of color are involved, the problem of
determining the comparative intensities of the lights becomes com¬
plicated with physiological and psychological difficulties of the most
serious character.

In the studies of visual acuity already referred to, especially
those of Uhthoff and Konig and of Broca and Laporte, these diffi¬
culties, which are generally recognized, were either entirely ignored
or met in a wholly inadequate matter. Konig states that no attempt
was made to reduce the intensities of the different colors to a common
basis, and in Broca and Laporte's work the photometric measure¬
ments seem to have been made in the loosest possible way.

In the present study the complexities of color vision have been
kept constantly in mind, and the determination of the comparative
intensities of the lights of different colors has been regarded as the
most important phase of the work. It is the purpose of this part of
the report to state briefly the difficulties involved in color photometry
and the various methods proposed for their solution, following this
by an account of the method adopted in the present investigation,
with arguments in support of the accuracy of the method.

Difficulties of a physiological character involved in estimating the
comparative intensities of lights of different colors arise from the

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