70 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, No. 5 
tion of the relative effectiveness of radiant energy in different parts 
of the spectrum in producing the sensation of brightness under certain 
conditions. This paper bears directly upon the problems which I 
wish to discuss later, but first it is desirable to consider some matters 
of nomenclature and units. 
DEFINITIONS AND UNITS 
Light—The word “‘light”’ itself is used in many different senses, 
but for the present purpose I wish to restrict it to a specific one. 
For this I must perhaps beg the indulgence of some of our members. 
I know there are some who cherish a little of the ancient sun-wor- 
shippers’ reverence for light as the mystic agency which stirs into 
being the processes on which all life depends, and which surrounds 
us with warmth and beauty. I trust, however, that a prosaic limi- 
tation of the term for our present purposes will not lessen anyone’s 
enjoyment of the beauties of the sunset, weigh down the buoyancy 
which an azure sky gives to our spirits, dim our appreciation of the 
iridescence which the oil film shows in lowly places, nor blind us to 
any other of the thousand and one glories which light provides for 
our enjoyment. If we are to consider light in any quantitative sense 
we must agree on some precise definition. 
The definition which I propose to use may be stated as follows: 
“Tight is radiant energy evaluated in proportion to its ability to 
stimulate the sense of sight.’”’ With respect to the ancient question 
whether there is then any light when no eye is present to perceive it, 
this definition is in away a compromise. The physical radiant energy 
is light, but how much light it is can only be told by applying coefficients 
depending on the properties of the visual apparatus. According to 
this definition ultra violet and infra red radiation are therefore not 
“light.” In order to emphasize the similarities of radiations of dif- 
ferent wave length it is perhaps allowable to follow Tyndall’s example 
in speaking of ‘Light, Visible and Invisible,” but for precise discussion 
there are good reasons for adhering to the stricter definition which 
brings in the coefficient of sensation. 
There are in fact many authorities who would go further and say 
that the term “light”? can properly be used only for the sensation 
aroused by radiant energy, but this can never be a quantitative 
definition, because we can not measure sensation except in a roughly 
approximate way. The sensation produced depends on the state of 
the physiological apparatus involved, so that the amount of sensation 
produced by radiation of the same kind and amount is widely different 
