686 Prof. McLennan and Mr. Dearie on the Infra-red 



that even with this lamp the current through it steadily- 

 decreased for the first ten minutes after the arc was struck. 

 This was brought about by an increase which took place in 

 the resistance of the mercury vapour when the temperature 

 of the lamp was rising. The following table and the curve 

 in fig. 1 (PI. XV.) show the variation of the current with 

 the elapse of time in a typical case : — 



Time. 



Current. 



min. 



6'4 amps. 



min. 50 sees. 



61 



1 „ 35 „ 



5-9 



2 „ 20 „ 



5-7 „ 



3 „ 10 „ 



545 „ 



3 ., 45 „ 



5-1 



4 „ 45 „ 



4-75 „ 



5 „ 40 „ 



4-4 



6 „ 35 „ 



413 „ 



7 „ 50 „ 



400 „ 



9 „ 10 „ 



3-75 „ 



10 ,, 10 „ 



372 „ 



11 „ — 



3-7 



i 



In all experiments care was taken to see that the lamp 

 was in the steady state before measurements were made on 

 the intensity of the radiation. 



The form of spectrometer used was one designed and con- 

 structed by the Adam Hilger Co. It is shown in fig. 2 and in 

 diagram in fig. 3 (PL XIV.). The energy measurements were 

 made with a sensitive Rubens thermopile shown in fig. 4, in 

 conjunction with a very delicate Paschen galvanometer made 

 by the Cambridge Scientific Instrument Co. The radiation 

 from the lamp was allowed to fall upon a large concave 

 mirror having a diameter of 19 cm. and a focal length of 

 30 cm., which brought it to a focus on the slit Si. From 

 this slit the rays passed to the nickel-steel concave mirror 

 Mx, thence through the rock-salt prism P to the plane nickel- 

 steel mirror M 2 . From this they were reflected to the con- 

 cave nickel-steel mirror M 3 , and by it they were brought to 

 a focus on the linear junctions of the thermopile at T, which 

 was placed immediately behind the slit S 2 . The prism and 

 piano mirror were mounted on a table which rotated about 

 the point A. By turning this table tli rough a small angle, 

 any desired part of the spectrum could be brought to a focus 

 at S 2 . The rotation was produced by the motion of a helical 

 drum attached at D, which was calibrated in wave-lengths 



