68 A. IV. Stewart on 



conditions. The views which are about to be described are due 

 to Sir Norman Tjoclcyer ; and although they are not accepted 

 in toto by the scientific world, they probably contain more than 

 a gr-ain of truth. 



When investigating the emission spectra of metals, Lockyer 

 found that the character of the spectrum emitted by an element 

 depended to a lai'ge extent upon the temperature at which the 

 metal was examined. Thus in a gas-flame having a temperature 

 of a few hundred degrees, a certain metal gave a characteristic 

 spectrum ; in the electric arc, at a temperature of 3000C, a 

 different spectrum was exhibited by the same metal ; whilst^ 

 finally, in the electric spark, which is supposed to have a 

 temperature of some 12000"C., the metal shows yet another 

 type of spectrum. 



Lockyer's examination of star spectra suggested to him that 

 among the stars there are bodies at all sorts of different 

 temperatures ; and he began to classify the stellar species into 

 groups according to this property. At first sight it might appear 

 to be impossible to determine which of two stars was the hotter; 

 but the matter is not so difficult as it might seem at first sight. 

 A simple example will make the matter clearer. Suppose that 

 the complete spectrum were spread out by a prism as shown 

 below : — 



Heat Kays^Red — Orange — Yellow — Green — Blue — 

 Indi(40— Violet — Ultra-Violet. Now assume that a piece of 

 metal, such as a poker, is gradually heated. In the first place, it 

 begins to emit heat rays while still remaining black to our eye. 

 Next, it gradually turns red, showi]ig that the red part of the 

 spectrum is being radiated from it. With a further increase of 

 temperature, its colour becomes lightei-, because it is now 

 radiating orange as well as red rays. A higher temperature 

 makes it glow bright red, for at this stage it is throwing out every 

 variety of ray from the heat-rays down to yellow ; and the joint 

 action of all these produces on our eye the sensation of bright-red. 

 Finally, if we still further raise the poker's temperature, it 



