28 



INFRA-RED REFLECTION SPECTRA. 



plates by means of soft wax, the plane faces being, of course, placed over 

 the openings in the plates. In case less than four reflecting surfaces were 

 available, aluminum mirrors were placed in the remaining holders. A 

 short focus mirror projected an image of the slit, S 2 , upon an improved 

 iron-constantan thermopile of 20 junctions. The wire used in the ther- 

 mopile was only 0.075 mm - * n diameter. This eliminated heat conduction 

 and there was no drift of the zero. The galvanometer suspension weighed 

 about 10 mg., which eliminated the effect of earth tremors. This, how- 

 ever, necessitated lengthening the period to gain great sensitiveness. The 



km. 19. Wire-grating calibration. 



Z8 



3ZM 



thermopile was in a metal case, wrapped in felt, and the complete outfit 

 was perfectly steady. It was, therefore, possible to increase the period of 

 the galvanometer to 20 to 30 seconds (single swing) when its sensitiveness 

 was f=5Xio~ n ampere on a scale at 1 m. The resistance of the ther- 

 mopile was 8.9 ohms. The galvanometer resistance was 5 ohms, from 

 which it was computed that a deflection of 1 mm. = 7X io~ 7 C. for a scale 

 at 1 m. 



From this it will be seen that the temperature sensitiveness of the 

 instrument was as great as, and in some instances greater than, in previous 

 investigations. Hence, it appears that in numerous cases the absence of 

 reflection bands is to be attributed to some property in the material rather 

 than to a fault in the instruments. 



The whole apparatus was, of course, thoroughly protected from stray 

 light by providing numerous black pasteboard screens. 



