48 E. L. Nichols— Optical Method for the 
image coincided. The spectrum formed by the two combined 
was much brighter than that of the flame alone, and it was 
easy, having measured the increase of intensity due to the 
image, to calculate how much of light reflected by the mirror 
and falling upon the flame, was absorbed, and how much 
allowed to pass through it. In making this calculation it was 
necessary to know: the reflecting power of the mirror, the 
transmitting power of the lamp chimney, and the relative in- 
tensity of the spectrum of the flame to that of the flame and 
image combined. 
First of all the transmitting power of the lamp-chimney was 
investigated. The lamp having been provided with an exactly 
similar chimney, the spectrophotometer adjusted and the spec- 
trum of this lamp flame having been compared with that of the 
flame lighting the upper half of the slit, the chimney to be ex- 
amined was suspended in the path of the rays between the 
flame and slit, whereupon the spectra were again measured. 
found that the chimney permitted the passage of 08573 of the 
light. Then the mirror was taken in hand The lamp being 
moved a few centimeters to one side and the mirror turned 
until the image of the flame occupied the former position of the 
flame itself; the intensity of the resulting spectram was meas- 
ured. This gave as illuminating power of the image, 0°6509 of 
the flame’s intensity. The lamp was then restored to its place 
. before the slit and the mirror readjusted until those portions of 
the flame and image which appear in the spectral image coin- 
cided. The spectrum of this double source of light was then 
measured, and by repeated intervention and withdrawal of a 
black screen between the lamp and mirror, compared with the 
spectrum of the flame above. I found the ratio of flame and 
image combined to the flame alone, to be 1°2075: 1. Had both 
flame and lamp-chimney been perfectly transparent, this ratio 
would have been 1°6509:1. The effect of the lamp-cylinder 
being eliminated the remaining difference is naturally to be 
attributed to the absorptive capacity of the flame; and we find 
A = 0°6482. 
To determine the value of A for platinum at the temperature 
in question, it only remained to compare the radiation of a 
glowing platinum wire with that of the flame. The wire hav- 
ing been given a temperature of 1650° (Pt. thermometer) for 
which the leucoscope* showed that the quality of the light 
emitted corresponded precisely with that from the petroleum- 
flame, the intensity of its spectrum was measured and found 
as compared with that of the flame to be as 1°198: 1. 
The value of A for platinum at this temperature is accord- 
ingly, A = 0°7597. 
* For the method by which the platinum wire was made to glow, and for a 
description of the leucoscope, see Paper I. 
