356 PHYSICS. 



bridge, aud the current from one or more Daniell cells is sent througii 

 them. When the two currents are equal the needle of the galvanometer 

 is unatfected. But when radiant energy falls on one of the systems 

 of strips and not on the other, the current passing through the first is 

 diminished by the increased resistance of the metal due to the rise in its 

 temperature. As the second remains unaltered, the needle is deflected. 

 Moreover, owing to the thinness of the strips, they take up and part with 

 their heat far more promptly than the thermopile, thus giving a much 

 greater rapidity of working. Eesults are given showing the extreme 

 sensitiveness and reliability of the instrument. For the first time it 

 has been possible to make actual measures of the distribution of heat 

 in the diffraction spectrum. This was done by Langley with the bolom- 

 eter, using a Eutherford plate of 681 lines to the millimeter, ruled on 

 speculum metal. The spectrum was 20 centimeters long and 8 milli- 

 meters wide, so that the balance received nearly homogenous rays. 

 The extremely" minute amount of heat received was found sufficient to 

 give a galvanometer deflection of some hundred divisions ; and this 

 where thermopiles have failed to detect anything. The deflections ob- 

 served for different wave-lengths were: for ;/=.00035. a=12', y=.O0O4:, 

 a=55; y=.0005, a=207 ', y=.0006, a=2AQ; ^=.0007, ar= 198 ;>=.0008, 

 a=l2d', y=xmd, a=80; y=.0010, a=58; y^.OOll, a=4.1. The max- 

 imum deflection for heat, then 246, corresponds to a wavelength of .0000, 

 that of D being .00059, giving the conclusion that the heat maximum in a 

 normal spectrum is not in the ultra-red, as has been supposed, but is in 

 orange, near I), the heat and light curves agreeing very closely. The 

 value of the instrument is obvious. {Am. J. Sci., March, 1881, III, xxi, 

 187; Proc. Am. Acad., January, 1881, xvi, 342; iVafJire, November, 1881, 

 XXV, 14.) 



Puluj has devised an ingenious experiment to prove that radiant 

 matter consists of particles separated from the electrode by the elec- 

 trical action. The cathode of a vacuum tube was covered with chalk. 

 It exhibits phosphorescence of an orange-yellow color, while in a short 

 time the wall of the tube becomes covered by a very delicate layer of 

 chalk without losing its clearness and transparency. The deposit phos- 

 phoresces like chalk, and has led to the suggestion which he makes, 

 that the phosphorescence of a yellow color observed on metallic ca- 

 thodes is caused by the phosphorescence of the oxides covering the 

 metal. {Mature, March, 1881, xxiii, 442.) 



The photophone and its results, discovered by Graham Bell and 

 Tainter (see report for 1880), have awakened a very general interest. 

 Kayleigh has given a discussion of the question whether the unelectrical 

 sounds produced by the simple impact of intermittent radiation upon 

 thin plates of various substances can be accounted for by the heat pro- 

 duced. He finds that if a plate of iron 6 centimeters in diameter be 

 exposed to an intermittent beam of sunhght at 250 vibrations per sec- 



