460 E. L. Nichols—Character and Intensity of the 
In table V the intensities of the various wave lengths for all 
the temperatures in question are expressed in terms of the 
intensities of corresponding wave lengths of a similar spectrum 
of constant but unknown temperature. For each individual 
ray, the meastirements give the change of intensity resulting 
rom a given change of temperature; but since the relative 
intensity of the various wave lengths of the spectrum of con- 
stant temperature are unknown, it is impossible to compare rays 
of different wave length with each other. 
Before describing the way in which all the above results 
were reduced to a common basis for the purpose of comparison, 
a clear definition of the expression ‘intensity of ray” as us 
in this paper is desirable. : 
It is first of all essential to distinguish between the intensity 
of the ray itself and the intensity of its various effects upon 
bodies upon which it may fall. It is usual to define as the 
intensity of the ray itself, its energy of vibration or the square 
of the amplitude of vibration. Kirchhoff however (§ 2 of his 
above mentioned treatise), defines as the energy of the ray pass- 
ing the openings of the screens S, and S, (fig. 1) the increase 10 
a unit of time of the vis viva of the ether behind the second 
screen. This definition is the more appropriate to the case at 
hand. Suppose that for the opening 8, a black body be sub- 
stituted. In accordance with the principle of the Conservation 
of Energy, as expressed in the usual equation, 
T,-T,=/">(X de} Vdy+Zde) =U, (7) 
to 
where T, and T, are the energy (lebendige Kraft) betore and 
after the interval of time, and U, denotes the work performe@; 
the increase in energy in the body equals in the unit of time, 
consider this work as the measure of the intensity of the we 
itself. Suppose further, the body be molecularly so constitute 
produced may in this case be taken as the measure of the pose 
sity of the ray itself. This heat, as denoted by the change © 
temperature of the body, is the result of what is termed the 
intensity of the thermal action, or the thermal intensity of the ee" 
: The chemical* and optical intensities of the ray stand in as ye 
_ * Clerk Maxwell, Theory of Heat, i 
p. 240, offers a very in 
a —— ature of the chemical action of light. “It is probable thet 
a produces the photographic effect, it is not by its energy 
_ the chemical compound, but rather by a well-timed vibration of the molecules 
