30» uo. W. Draper—New form of Spectrometer. 
Art. I11.—On a new form of Spectrometer, and on the distribu- 
tion of the intensity of Light in the Spectrum ; by JoHn WIL- 
LIAM Draper, M.D., President of the Faculty of. Scinscnt in @ 
the University of New York. 
I HAVE invented a “Soe: tities which I think will opena _ 
new and interesting field to those who are engaged in spectrum 
se Sa | 
e ordinary spectroscope is ne with the frequency of 
ether-vibrations or wave lengths. This, which Iam about to 
describe, has a different function. It deals with the intensity : 
or imme of light. 
epends on the well known optical principle that a light 
ices invisible when it is in presence of another light about 
sixty-four times more brilliant. 
In some researches, published by me in 1847, on the produc- 
tion of light by heat or the incandescence of bodies, I used this 
method as a photometer, wi became sensible of its value. — 
The memoir in which those experiments are related may be _ 
found in my recently published ‘Scientific Memoirs,” page 23. 
~ Having also published in 1872 a memoir on the distribution 
of heat in the prismatic spectrum, and shown that the cause 
of its increasing intensity from the more to the less refrangible 
regions is due to the compression of the colored spaces that 
earn teers 8 takes place, owing to the action of the prism 
tself, but having failed to obtain satisfactory measures in the 
ane of the diffraction spectrum, in which such compressi ; 
condensation does not occur, I was led to reflect whether bettet 
success might not be secured by attempting to measure the 
relative intensity or distribution of the li 
Admitting what is commonly received as true, that the yel- 
low is the brightest of the colored spectrum spaces, and that — 
the luminous intensity diminishes from that in both directions, 
aga and below, I supposed that if such a spectrum was brought 
n presence of an extraneous light, the illuminating power of 
which could be varied at pleasure, that after the red and gee '4 
orange on one side, and the green, blue, indigo and violet, on ~ 
the other, had been extinguished, the yellow would still sangre : 
n the midst of the surrounding illumination. On making the 
riment it turned out different 
or the sake of clearness of description I will call this 
extraneous light, from the function it has to discharge, the ex- 
tinguishing light. : 
There are — sen Les bys coe ree rinciple above — 
indicated may be carried Raver ral of these — 
LThave tried, ia have robere: d the folle ebeetig a  oneaiiaat one. 
