S. P. Langley — Observations on Invisible Heat-Spectra. 7 



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 bolo- 

 meter was then given directly by the circle reading to 10 r/ . 



The bolometer used was 2 mm wide, and consequently sub- 

 tended 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 trans- 

 mission of rock-salt in different parts of the spectrum, and for 

 this purpose a plate of polished rock-salt, whose thickness was 

 9 , l mm , 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 radiations from the salt itself, when the screen was withdrawn 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. 



Deflection with Transmission 



rock-salt plate interposed. of plate. 



Deviation. 



Deflection 



40° 



30' 



12 



40 



00 



72 



39 



30 



214 



39 



00 



364 



38 



30 



420 



38 



00 



365 



37 



30 



269 



37 



00 



196 



36 



30 



137 



36 



00 



96 



35 



30 



62 



35 



00 



48 



34 



30 



29 



34 



00 



26-5 



33 



30 



18 



33 



00 



10.5 



360 

 251 

 122 



63 



27 

 14 



•857 



•933 



•891 



•02 



•931 



•778 



The "transmission" of the plate of rock-salt (here uncor- 

 rected for heat lost by reflection at the anterior surface) has 

 been shown by subsequent experiments to very slightly 

 diminish for extreme infra-red heat rays in the Leslie cube spec- 

 trum ; but to remain so nearly constant through the range of 

 these experiments, as to show that the present approximate 

 values need no correction on this account. More exact ones 

 will be given in a subsequent memoir. 



The following series was then taken for fixing more accu- 

 rately the position of the maximum : 



