S. P. Langley—The Actinic Balance. 195 
The instrument in its present condition has been used with- 
out any lamp-black upon the steel strips, for fear that its well- 
<nown hygrometric qualities might injure them by causing 
rust, but this objection will not apply to the platinum. The 
writer, however, has grave doubts about the advisability of 
treating the lamp-black as absorbing all heat-rays indifferently, 
although such a statement of its capacity is given by Melloni, 
and is very widely adopted on his eminent authority. A spe- 
cial investigation into the absorptive power of lamp-black will 
probably form a part of the present series of researches under- 
taken here. In its unblackened condition the instrument ap- 
pears to be, roughly speaking, from 5 to times as sensi- 
the needle’s period of vibration brief, the probable error is of 
course less. These are general considerations, which affect 
alike the thermopile and actinic balance, but the latter instru- 
umstances, in repeating some of Melloni’s experiments on radi- 
A few observations are given here just as they are found in 
the note books, and which probably represent fairly the ave- 
rage accuracy of the instrument in its present condition, except 
that the source of heat being the sun, whose radiation varies 
m moment to moment from atmospheric causes, the probable 
error is larger than it would have been with a constant source 
of heat. The galvanometer used in the repetition of Melloni’s 
