MEASURING HEAT FROM STARS 443 



from all the stars which at any moment can fall upon 1 sq. cm. of the 

 earth's surface is so small that it would have to be absorbed and con- 

 served continuously for a period of 100 to 200 years in order to raise 

 the temperature of 1 gram of water 1 degree centigrade. Evidently the 

 incoming stellar radiation can contribute but little in retarding the 

 cooling of the earth. For the measurements of nocturnal radiation, 

 which is usually a loss of terrestrial radiation into space, indicate that 

 for a lampblack surface the outgoing radiation may be as high as 0.1 

 the incoming solar radiation.. The emissivity of the materials forming 

 the earth's surface may be much lower than this value. Nevertheless, 

 it is much greater than can be compensated for by a continuous incom- 

 ing of stellar radiation. The temperature of the earth must therefore 

 ultimately tend towards the absolute zero of temperature. 



V. Some Astroeadiometric Problems that Await Solution" 



A complete . statement can not be made of the problems in stellar 

 radiation that require investigation. They stare one in the face which- 

 ever way one turns. Eealizing the inadequacy of the radiometric appa- 

 ratus now available one must sit with eyes closed in order not to become 

 impatient with existing conditions, both as regards the production of 

 radiometric apparatus sufficiently sensitive to make the measurements 

 and as regards financial assistance which is necessary to carry on the 

 work. It is not a work that can be " cleaned up " in a season. It will 

 require years of painful, nerve-racking toil in order to accomplish any- 

 thing of worth. 



While it is hoped that the present investigation will make available 

 to the astronomer one more instrument for the investigation of celestial 

 objects, it is desirable to emphasize that, from the insensitive nature 

 of the instrument, the astronomical application can not be very wide 

 as compared with the spectrograph. However, its physical properties 

 are such that, in a limited field, it can be employed in attempting the 

 solution of some of the most fundamental questions in astrophysics. 

 Take for example the question of the emissivity of blue stars as com- 

 pared with red stars. The general conclusion appears to be that blue 

 stars are at a higher temperature than red stars, and that the emissivity 

 of the red stars is higher than that of the blue stars. The higher 

 emissivity of the red stars would be attributable to a marked change in 

 the distribution of energy in the spectrum, brought about by a change 

 in the physical condition of the stellar surface. 



"With the rather insensitive radiometric outfit used in the present 

 investigation it was shown that the total radiation received from a red 

 star is two to three times that of a blue star of the same photometric 

 brightness. These observations should be extended. Another field of 

 astroradiometric research is the measurement of the radiation from 



