OP ARTS AND SCIENCES. 349 



diminished by the increased resistance ; and, the other current remain- 

 ing unaltered, the needle is deflected by a force due to the battery di- 

 rectly, and mediately to the feeble radiant heat, which, by warming the 

 strips by so little as ytyoiJTT ^^^ degree Centigrade, is found to produce 

 a measurable deflection. A change in their temperature of ttjoVittt 

 degree can, I believe, be thus noted; and it is evident that from the 

 excessive thinness of the strips (in English measure from ^^^ to 

 ^^^^^ inches thick) they take up and part with the heat almost in- 

 stantly. The instrument is thus far more prompt than the thermopile ; 

 and it is also, I believe, more accurate, as under favorable circumstances 

 the probable error of a single measure with it is less than one per cent. 

 When the galvanometer is adjusted to extreme instability, the probable 

 error of course is larger ; but I have repeated a number of Melloni's 

 measurements with the former result, 



I call the instrument provisionally the "Bolometer," or "Actinic 

 Balance," because it measures radiations and acts by the metl^d of 

 the " bridge " or " balance," there being always two arms, usually in 

 juxtaposition, and exposed alike to every similar change of tempera- 

 ture arising from surrounding objects, air-currents, etc., so that the 

 needle is (in theory at least) only affected when radiant heat, from 

 which one balance-arm is shielded, falls on the other. 



Its action, then, bears a close analogy to that of the chemist's bal- 

 ance, than which it is less accurate, but far more sensitive. The 

 sensitiveness of the instrument depends, as has been explained, upon 

 the amount of current used. With the current which experience has 

 recommended, as leaving a very steady galvanometer needle, this 

 sensitiveness appears to be from ten to thirty times that of my most 

 delicate thermopiles, area for area ; but I consider this quality valuable 

 only in connection with its trustworthiness as a measurer, always re- 

 peating the same indications under like conditions. 



The working face of the instrument, as I have used it, exposes about 

 one half of one square centimetre to the source of radiant heat (it can 

 easily be made of any other size, larger or smaller) ; and the strips are 

 shielded from extraneous radiations by the most efficient precautior-a 

 which a rather long and painful experience in guarding against them 

 has taught me. 



Description of Figure 1. 



There are two disks of hard rubber, each 30 mm. in diameter and 3 

 mm. thick in the thickest part. Each has a concentric opening 8 mm. 

 square. The first has four hollow brass cylinders, a, c, b, d, and the 



