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



Wave-length. 



Width of spectrum. 



054 fx 



0-02 n 



0-58 „ 



0-03 „ 



0-82 „ 



008 „ 



loO „ 



0-28 „ 



200,, 



062 „ 



300 „ 



0-76 „ 



400 ., 



68 „ 



5-00 ., 



054 „ 



6-00 „ 



0-46 „ 



7'00 „ 



0-40 „ 



8 00., 



036 „ 



io-oo „ 



032 „ 



between these readings, and the wave-lengths could be 

 assigned to the various lines with an accuracy of probably 

 ±0-01 fi. 



IV. Observations. 



After the apparatus was carefully set up and found to be 

 in good working order, a set of readings was taken on the 

 energy spectrum of an Arons * amalgam lamp. The amalgam 

 in this lamp consisted of about 60 per cent. Hg, 20 per cent. 

 Pb, 20 per cent. Bi, J per cent. Zn, and -J per cent. Cd. The 

 lamp was run on the 110 volt direct-current circuit with a 

 suitable resistance in series. The energy curve is shown in 

 fig. 7 (PL XV.). Thirty-six distinct maxima were observed 

 in all between wave-lengths 0*70 fi and 3'0/x. Possibly as 

 many more could have been distinguished by repeated obser- 

 vations, but as there was no way of distinguishing which 

 were mercury lines and which were lines of the other metals, 

 further work with this lamp was abandoned. 



Readings were then taken, as stated above, with a lamp 

 containing only pure mercury. The maxima from a large 

 number of sets of readings were compared, and the regions 

 which contained constantly recurring maxima were carefully 

 examined so as to establish the exact position of each maxi- 

 mum as closely as possible. In the same region as that 

 investigated by Paschenf, the positions of nine lines were 

 determined, and these accorded fairly well with the wave- 

 lengths given by him. As was expected, in regions where 

 the latter gives two or three lines in close proximity, only 

 one maximum, corresponding to the mean wave-length, was 

 recorded in our measurements. Corresponding to the new 



* Arons, Ann. d. Phys. Band xxiii. (1906). 

 f Paschen, loc. cit. 



