MAR. 4, 1923 CRITTENDEN: MEASUREMENT OF LIGHT 71 
at different times. Moreover, at any given time, sensation is not pro- 
portional to the amount of the physical stimulus. In other words, 
in psychological effects two and two do not make four, and such effects 
can not well be used as a measure of other quantities. A condition 
somewhat analogous to this is represented by what the economists 
call the “law of diminishing returns.”’ For the individual ultimate 
consumer at a given time two apples are seldom worth twice as much 
as one, and yet no one would deny that they are twice as many. So 
in the case of light it would be quite impractical to say that the 
quantity is doubled only when the sensation is doubled. In actual 
practice we must have some name which designates the quantity, 
partly physical, partly physiological and psychological, which repre- 
sents an amount of radiant energy with due allowance for the useful- 
ness of that particular kind of radiant energy for purposes of vision. 
The most practical course is to use the term “‘light”’ as is done in 
common speech. 
It must be granted, however, that such expressions as “light is 
emitted from a lamp” or ‘“‘the light falls on the page’”’ are logically 
somewhat peculiar, since in these cases the actual phenomena involved 
are purely radiation, and the visual factor comes in only by a sort of 
mental juggling. We may perhaps imagine that as the radiation 
passes from lamp to book some spirit tags each element of it with a 
coefficient suitable to its frequency, which coefficient represents the 
magnitude of the effect which will be produced when the radiation 
reaches the eye. The thing which actually exists in space in definite 
amounts is radiant energy; the visual factor is obviously not really 
applied until this energy is absorbed by the visual apparatus and 
changed into a distinctly different form. In brief, light in this quanti- 
tative sense has no real physical existence. This fact is an argument 
against the proposals which have been made to simplify nomenclature 
by using the same names for units of light as for energy. It has been 
urged that the unit of light flux be that amount produced by one watt 
of radiant power at the most effective frequency, and that this unit 
also be called a watt. As an illustration of the practical confusion 
which would result from such a nomenclature it may be said that the 
lamp known as a ‘‘60-watt’’ at present would give about one ‘‘watt”’ 
of light flux. While this would serve to emphasize how far our 
practical illuminants fall below the most efficient monochromatic 
radiation, this advantage would hardly compensate for having two 
“watts” of entirely different kinds. Since the physiological-psycho- 
logica] sensation factor must come in in the translation of energy 
