THE WA VE THEORY OF LIGHT. 321 



Professor Langley giving as a fact that the light 

 from the moon drives the indicator of his sensi- 

 tive instrument clear across the scale, showing a 

 comparatively prodigious heating effect ! 



I must tell you that if any of you want to 

 experiment with the heat of the moonlight, you 

 must measure the heat by means of apparatus 

 which comes within the influence of the moon's 

 rays only. This is a very necessary precaution ; 

 if, for instance, you should take your Bolometer or 

 other heat detector from a comparatively warm 

 room into the night air, you would obtain an 

 indication of a fall in temperature owing to this 

 change. You must be sure that your apparatus 

 is in thermal equilibrium with the surrounding 

 air, then take your burning-glass, and first point 

 it to the moon and then to space in the sky 

 beside the moon ; you thus get a differential 

 measurement in which you compare the radiation 

 of the moon with the radiation of the sky. You 

 will then see that the moon has a distinctly heating 

 effect. 



To continue our study of visible light, that is 



Y 



