EXPLORING THE ATOM 



on this delicate apparatus they disturb its balance, 

 because the blackened disc absorbs the rays whereas 

 the bright disc reflects them. So wonderfully delicate 

 is the adjustment that a candle placed more than 

 one-third of a mile away turns the vanes of the instru- 

 ment through nearly one hundred scale-divisions. As 

 one-tenth of a single division can be readily detected, 

 it will be seen that a candle at this distance by no 

 means puts the implement to its fullest test. 



It is estimated that, were there no atmospheric 

 obstruction, the candle could be detected at a dis- 

 tance of sixteen miles. The face of an observer can 

 be detected at a distance of several miles; at two 

 thousand feet it turns the vanes through twenty-five 

 scale divisions. 



So every human countenance glows as a beacon 

 light, signaling out for miles in every direction only 

 one must be equipped with a radiometer if one would 

 note or heed the signals. 



Directed toward the sky, the radiometer proves 

 adequate to the task of registering the radiant energy 

 of the larger stars and planets. The experiments of 

 Professor Nichols have sufficed to show that the 

 radiation push of a star can not be definitely predi- 

 cated from observation of its luminosity. Thus it 

 was found that the planet Saturn has only about 

 three-fourths the thermal effect of the star Vega; the 

 star Arcturus produces three times the effect of Sat- 

 urn; the planet Jupiter more than six times as much, 

 relations quite different from the relative bright- 

 ness to the eye of these various bodies. 



Very recently Professor A. H. Pfund of Johns 



in 



