282 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 31 



about 4 times, 7 times, and 14 times as energetic as the " helium 

 rays." 



7. The radiation corresponding to the smallest annihilation process 

 that can take place — the suicide of a positive and negative electron — 

 is 350 times as energetic as the hardest gamma ray, or 35 times as 

 energetic as the " helium ray." 



This brings us to the tenth discovery, that of the cosmic rays. 

 These reveal : ^ 



1. A radiation, the chief component of which, according to our 

 direct comparison, is five times as penetrating as the hardest gamma 

 ray, which, with the best extrapolation we can make on the curve 

 connecting energy and penetrating power, means a ray 10 times as 

 energetic as the hardest gamma ray, precisely according to prediction. 



2. Special bands of cosmic radiation that are roughly where they 

 should be to be due to the formation of the foregoing abundant ele- 

 ments out of hydrogen, though (for reasons to be given presently) 

 no precise quantitative check is to be expected except in the case of 

 the helium rays. 



3. No radiation of significant amxount anywhere near where it is 

 to be expected from the annihilation hypothesis, thus indicating that 

 at least 95 per cent of the observed cosmic raj^s are due to some other 

 less energetic processes. 



4. A radiation that is completely independent of the sun, the great 

 hot mass just off our bows, and not appreciably dependent on the 

 Milky Way or the nearest spiral nebula, Andromeda, one that comes 

 in to us practically uniformly from all portions of the celestial dome, 

 and is so invariable with both time and latitude at a given elevation 

 that the observed small fluctuations at a given station reflect with 

 much fidelity merely the changes in the thickness of the absorbing 

 air blanket through which the rays have had to pass to get to the 

 observer. 



This last property is the most amazing and the most significant 

 property exhibited by the cosmic rays, and before drawing the final 

 conclusions its significance will be discussed. For it means that at 

 the time these rays enter the earth's atmosphere, they are practically 

 pure ether waves or photons. If they were high-speed electrons or 

 even had been appreciably transformed by Compton encounters in 

 passing through matter into such high-speed electrons or beta rays, 

 these electrons would of necessity spiral about the lines of force of 

 the earth's magnetic field and thus enter the earth more abundantly 

 near the earth's magnetic poles than in lower latitudes. This is 

 precisely what the experiments made during the last summer at 



* See articles by Millikan and by Millikan and Cameron, Phys. Kev., Dec. 1, 1930, and 

 Feb. 1, 1931. Also Nature, Oct. 24, 1931. 



