MIRACLES OF SCIENCE 



ments, chiefly at the solar observatory at Mt. Wilson, 

 to determine the "solar constant." He makes the 

 solar constant 1.92 calories per minute per square 

 centimeter of surface normal to the solar radiation 

 at the earth's mean distance; but he strongly suspects 

 that the radiation is variable to the extent of about 

 8 per cent, in the course of a few days. It is probable, 

 therefore, that the "solar constant" is not a constant; 

 and that our sun is a variable star. Knowledge of 

 this variability of the sun as a heat-giver will perhaps 

 ultimately be available in predicting weather condi- 

 tions here on the earth as influenced by sun spots 

 or other solar phenomena. 



But aside from these practical results, very great 

 interest attaches to the work done with the bolo- 

 meter, in that it enables the observer to detect and 

 measure the presence of waves of energy beyond the 

 visible spectrum. Indeed, it appears that an impor- 

 tant concentration of heat rays occurs in the dark 

 region below the deepest red, although in a normal 

 spectrum the greatest focus of energy is in the blue. 



Langley was able with the bolometer to chart this 

 infra-red region of invisible light, if the term be per- 

 mitted. He not only tested its gradations of heat 

 but showed that it is crossed by hundreds of char- 

 acteristic cool bands comparable to the dark lines of 

 the visible spectrum. 



Meantime it had been discovered that the rays of 

 light that chiefly affect the photographic plate are 

 those toward the violet end of the spectrum, and 

 extending into a region beyond the utmost visible 

 portion of the violet. It had long been known that 



