194 J. EF. Keeler—Absorption of Radiant Heat 
, 
made movable in its own plane through a small distance in 
every direction, and all possibility of direct rays from it reach- 
ing the image was removed by interposing a series of screens. 
The bolometer used, for which I am indebted to the kindness 
of Prof. Langley, had a resistance on each side of about 30 ohms, 
and exposed a surface o . It was fastened upon a small 
hinged lid, which, when shut, brought the working surface of 
the bolometer exactly opposite an aperture of the same size in 
the copper plate that formed the front surface of the sliding 
carriage, the bolometer in this position being otherwise com- 
pletely enclosed in a copper lined cavity. e galvanometer 
used in connection with the bolometer was a very small instru- 
ment on Sir Wm. Thomson’s plan with modifications by Prof. 
von Helmholtz. The mirror, on the back of which were cemented 
two minute bar magnets, was suspended by a single spider's 
thread 6™ long, so that there was no perceptible torsion. The 
deflections were observed by the telescope and scale method, 
and, as they never exceeded 10™ on the 126™ distant scale, 
educt 
eter aperture, heat from the source experimented upon was ad- 
mitted by pulling the cord, and the galvanometer gave a prompt 
