[mclennan-petrie] spectra OF HELIUM, ETC. 25 



3. That in identifying the wave-lengths obtained in vacuum 

 grating spectra of helium, and possibly also of hydrogen, care should 

 be taken to make due allowance for the possibility of wave-lengths 

 being recorded on the plates which have their origin in carbon or 

 mercury introduction into the discharge tube either during the 

 exhaustion of the spectrograph or the purification of the gases. 



The Physical Laboratory, 

 University of Toronto, 

 June 1st, 1921. 



