eae a lh 
— 
ad 
Rays emitted by Glowing Platinum. 447 
Es 
It is the purpose of this paper to describe a series of such 
researches, made in the Physical laboratory of Professor 
Helmholtz, at Berlin. 
The quantity I is (see preceding foot-note) a function of the 
wave lengths of the ray and the temperature of the radiating 
body. Its study, therefore, involves the measurement of the 
intensity of all wave lengths emitted by the source of light in 
question, at all temperatures for which the rays are of per- 
ceptible energy. 
_ The nature of the subject demands different methods of 
investigation for the study of the visible and of the invisible 
rays. The measurements to be described in this paper are 
confined to the visible rays, and the lowest temperature under 
consideration is that at which bodies begin to glow. 
wo platinum wires 100™ long and about 0-4™™ in thickness, 
served as sources of radiation. ach formed part of a powerful 
galvanic circuit, in which the current was produced by a 
Bunsen’s battery. The resistance of each circuit could be 
varied by introducing or withdrawing copper wire, after the 
ore of the Wheatstone’s bridge. One of these bridges 
Showed by the motion of a spot of light upon a screen, 
and a half meters distant, every change in the intensity of the 
current and, of course, in the temperature of the wire. ‘The 
galvanometer, when properly adjusted, was sufficiently delicate 
smaller changes of temperature 
than could be detected either by observing with the eye changes 
of color in the wire, or by studying with any known instrument 
the changes in the character of the light emitted. Quite as 
essential to success as the constant temperature of the wires 
during a single experiment, is the ability to reproduce in the 
es WW; Ws 
He casas dy to the 
Jere A denotes the ratio of the intensity of rays absorbed by the body to 
Whole intensity of the rays falling upon it. In other words, A is the capacity 
absorption of the body. 
