Transmission of Infra-red Rays and of Radiant Energy. 59 



The eyepiece being removed -from the telescope was replaced by an 

 adjustable vertical slit, immediately behind which was mounted a delicate 

 thermopile of 10 bismuth-silver elements.* The terminals of the thermo- 

 pile were connected directly to a Paschen galvanometer! by the deflection 

 of which the energy falling on the thermopile could be measured. The 

 whole telescope was insulated from radiant and convected heat by a 



4 



i l p 



N = Nernst lamp. 

 C = Condenser. 

 P = Preparation in trough. 

 S' = Slit. 

 C = Collimator. 

 W = Wave-length drum. 



P' = Constant deviation prism. 

 T = Telescope. 

 S = Slit. 



TC = Thermocouple. 

 V - - Vacuum flask. 



Pig. 1. — Plan of Infra-red Spectrometer. 



silvered vacuum flask, the mouth of which was closed by dry cotton wool. 

 The prism was of special dense flint, and the prism table was calibrated in 

 wave-lengths throughout the visible and infra-red regions to A, 20,000. The 

 collimator slit had specially curved jaws to compensate for the difference in 

 refraction suffered by an oblique ray compared with one falling normally 

 on to the prism surface. The condenser system was mounted on a long arm 

 which extended beyond and in a line with the collimator. The light source 

 was a single vertical Nernst filament, taking 100 watts, approximately. The 

 positions and focal lengths of the lenses forming the condenser system were 

 carefully studied, the principle employed in the construction being one that 

 had been found by one of us to be very valuable when applied to the 

 microscope.J The lens system consisted of two chief components. The first 

 component consisted of two separate lenses which collected the rays diverging 

 * ' Trans. Optical Soc.,' vol. 13, p. 179. 



t A Broca galvanometer was used for the earlier experiments. 

 \ 1 Roy. Micro. Soc.,' 1913, p. 365. 



F 2 



