ASTRO-PHYSICS 269 



Glass is opaque to the infra-red radiation also, 

 and here again the advantages of the concave 

 grating are manifest. The infra-red spectrum 

 was examined, chiefly by Professor S. P. Langley 

 of Washington, through the heating effects of its 

 constituent rays. Professor Langley used an 

 instrument called the bolometer, in which the 

 heating effects of different parts of the spectrum, 

 and consequently the position of the dark lines, 

 are determined by measuring the change in 

 electric resistance of a very thin strip of 

 platinum exposed to the radiation. This form 

 of platinum thermometer is extremely sensitive, 

 and the spectrum of the sun has been mapped 

 far below the limits within which the eye re- 

 sponds to the stimulus of light. And, in more 

 recent years, the invention of new processes has 

 carried photographic methods beyond the range 

 of the eye at this end of the spectrum also. 



Perhaps the most striking and interesting 

 results given by the combination of camera 

 and spectroscope are those obtained by the 

 determination of the change in the refrangi- 

 bility of light produced by relative motion of 

 approach or retrocession of the source of light 

 and the receiving station. Let us imagine that 

 waves are proceeding from some source which 

 remains at rest. A certain number of waves 

 reach an observer in one second. If, however, 

 the observer is approaching the source, it is 

 evident that, as he is going to meet the waves, 

 a greater number of them will reach him in one 

 second than when he was at rest. Similarly, if 

 the observer move away from the source, the 



