400 Prof. S. P. Langley on Invisible Heat-spectra 



trometer (described in the Amer. Journ. of Science, vol. xxv. 

 1883, and the Mt. Whitney Report, chap. xi.). 



To illustrate the use of the apparatus, we give below in 

 detail the observations of March 20, 1885, for determining 

 the form of the energy- curve in the spectrum of a Leslie 

 cube at 178° C. The temperature of the room was —7° C; 

 so that the excess of the temperature of the cube over that of 

 the room was 185° C. 



The reading of the circle was made 0° 0' 0" when the spec- 

 trometer-arms were in line, and the direct image of the slit 

 fell on the bolometer. The prism was then placed on its 

 table, the automatic minimum deviation apparatus connected, 

 and the prism set to minimum deviation by a sodium-flame 

 held in front of the slit. The deviation of the ray falling 

 upon the bolometer was then given directly by the circle 

 reading to 10". 



The bolometer used was 2 millim. wide, and consequently 

 subtended an angle of about 20' in the spectrum. After 

 adjusting the prism, the slit was opened to the same width. 

 A secondary object of the experiment was the determination 

 of the transmission of rock-salt in different parts of the 

 spectrum, and for this purpose a plate of polished rock-salt, 

 whose thickness was 9*1 millim., was interposed at the slit, 

 after each deflection obtained in the ordinary manner, the 

 plate being allowed to remain in each case till the bolometer 

 had registered the heat due to radiations from the salt itself, 

 when the screen was withdrawn and the radiations from the 

 Leslie cube allowed to pass through it. 



The results are given below in the form of a table. 



Deviation. 



Deflection. 



Deflection with 



rock-salt plate 



interposed. 



Transmission 

 of plate. 



40 30 



12 







40 00 



72 







39 30 



214 





. 



39 00 



364 







38 30 



420 



360 



•857 



38 00 



365 







37 30 



269 



251 



•933 



37 00 



196 







36 30 



137 



122 



•891 



36 00 



96 







35 30 



62 



63 



1-02 



35 00 



48 







34 30 



29 



27 



•931 



34 00 



26-5 







33 30 



18 



14 



•778 



33 00 



105 







