in the Infra-red Region of the Spectrum. 739 



formed the top and bottom of the prism. The latter was in 

 communication with a U-tube manometer and a pump. The 

 temperature was taken by means of a thermometer in contact 

 with the prism reading to tenths of a degree. The height of 

 the mercury column in the manometer was read by a cathe- 

 tometer marked in tenths of millimetres. 



The arrangement of the apparatus is shown in the diagram 

 below : — 



The source of energy used was the Nernst lamp N. The 

 concave mirror m and the plane mirror m' were used so that 

 the image of the filament fell on the slit I. Between m and 

 m' was a metal diaphragm with doable walls provided with 

 a shutter. The rays from I were diverted by means of 

 another concave mirror M to fall on the rock-salt prism a, 

 set in minimum deviation for all radiations by Wadsworth's 

 method. This prism had been used previously by Professor 

 F. Paschen* in his experiments on dispersion by rock-salt. 

 The gas prism itself was standardized for the infra-red radia- 

 tions by means of it. The rays emerging from a wire were 

 focussed by M' on I', which was the second slit of the spectro- 

 scope IMM'L'. The spectroscope could be read to T g-g- of a 

 second. The gas prism A was placed one metre from I' and 

 was set in the position of minimum deviation. Immediately 

 behind it was a metallic mirror M'' with radius one metre. 

 This helped to form an image of V at a point B on the same 

 side of A as I, the radiations thus passing twice through the 

 prism. At B was a thermopile connected with a galvanometer 

 of the Paschen type. The thermopile could be displaced by 

 means of a Zeiss travelling microscope to which it was 

 attached. This enabled the linear displacement {Ad) to be 

 read to yoVo mm * Since the distance M" B was known 



* F. Paschen, Ann. der 2fy& iv. Bd. 20, p. 120 (1908). 



