80 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES’ VOL. 13, No. 5 
THE FUNDAMENTAL QUESTION OF LIGHT EVALUATION 
Limited Significance of Measurements.—It may be considered that 
we have for practical purposes fairly complete and satisfactory para- 
phernalia for light measurements. It should be noted, however, that 
so far nothing has been said about the properties of the eye which 
was, by the definition of light, made the arbiter as to what is “light” 
and what is not. Most of the measurements that have been men- 
tioned might be made by a bolometer or other radiometer as well 
as by the eye if it were found convenient to do so, since it has been 
tacitly assumed that the lights to be compared would be alike in 
quality. So far, therefore, we have dealt with definite physical rela- 
tions wherein the attainment of accurate values is a straightforward 
problem of laboratory technique. Personal and instrumental errors 
of course, come in, but in each case there is a definite and correct 
answer to the problem, and this correct result can be approximated 
more and more closely in proportion to the skill and patience shown 
by the observers. Unfortunately, however, this condition does not 
apply to many of the so-called “‘measurements” of light which we 
have to make. 
I began by defining light as radiant energy evaluated in proportion 
to its ability to stimulate the sense of sight, but we must ask what 
‘sight’? means when we wish to compare lights that differ distinctly 
in spectral composition. Does it mean ability to see fine details 
distinctly, to distinguish differences of light and shade, or does it 
mean the ability to take in a general perception of our surroundings? 
Of course, sight means all of these combined, but unfortunately for 
our “measurements” of light, when we have to deal with lights of 
different quality these different functions of sight assign different 
relative values to them, and in order to get any start at all on quanti- 
tative data we shall have to say more exactly what we mean by the 
sense of sight. 
So long as the lights dealt with are of about the same color it is 
easy to find the relation between them, and it has been generally 
agreed that measurements shall be made by some device which obtains 
an equality of brightness on two white surfaces side by side. As a 
natural development the same procedure has commonly been used for 
lights differing in color, and following this custom we may assume 
“sense of sight” for the present purpose to mean “‘sense of brightness.’’ 
But when we have to deal with lights of considerable color difference 
it becomes doubtful what is meant by equal “brightness.” The 
