230 



Sir Norman Lockyer. 



[Jan. 30, 



clearly stated by Sir George Stokes* in 1876, in the following words, 

 " When a solid body such as a platinum wire, traversed by a voltaic- 

 current, is heated to incandescence, we know that as the temperature 

 increases, not only does the radiation of each particular refrangibility 

 •absolutely increase, but the proportion of the radiations of the different 

 refrangibilities is changed, the proportion of the higher to the lower 

 increasing with the temperature." 



This question was also investigated by Melloni and Crova ; and in 

 recent years exact determinations of the law of increase have been 

 made by Lummer, Paschen, and others. Melloni showed! experi- 

 mentally that the maximum radiation moved towards the more 

 refrangible end of the spectrum as the temperature increased. Crova 

 made use of this fact in determining the temperatures of various 

 incandescent light sources, and was one of the first to suggest J that the 

 method was applicable to the determination of the temperatures of the 

 sun and stars. 



3. The Observational Conditions. 



The kind of spectroscope to be used and method of observation to be 

 followed were indicated by the following considerations. 



In order to utilise the effect of temperature changes to the full it was 

 necessary to record the red end of the spectrum as well as the ultra- 

 violet, as only in this way could the relative changes in intensity be 

 recorded, hence it ivas desirable to employ only a small dispersion. 



In addition to the natural differences photographed in the ultra- 

 violet, artificial differences due to the absorbing effect of our 

 atmosphere — which, even when clearest, is more or less opaque to the 

 ultra-violet radiations — might be introduced, therefore it was considered 

 advisable, in order to eliminate the effects of atmospheric absorption, 

 to obtain the spectra of any two stars to be compared whilst they were 

 at approximately the same altitude. Further, to avoid the many pit-falls 

 to which those who compare photographs taken on plates of unequal 

 sensitiveness, and differently exposed and developed, are liable, it was 

 obviously important that any spectra to be compared should be 

 obtained on the same plate in order to secure identical plate sensitive- 

 ness and development. 



Again, in order to secure similar optical treatment it was arranged 

 to photograph both spectra near to the optical axis of the camera ; 

 thus they are near together in the centre of the plate. 



I am sorry to say this work has been considerably delayed by the 

 long time taken in preparing a new camera and optical parts suitable 

 for the research, as above defined, and latterly by a long spell of bad 

 observing weather. 



* ' Roy. Soc. Proc.,' vol. 24, p. 353, 1876. 

 f Taylor's 1 Scientific Memoirs,' vol. 1, p. 56. 

 X ' Comptes Rendus,' vol. 87, p. 981. 



