164 Master Minds of Modern Science 



belief that the rays emanated from certain groups of stars 

 at vast distances from our system. 



The War intervened, and it was not until the end of 

 1921 that Dr R. A. Millikan began to investigate these 

 1 cosmic ' rays. 



Working with another scientist named Bowen, he sent 

 up electroscopes to a height of nearly ten miles. Of course 

 Millikan did not go up himself, for in the bitter cold and 

 rarefied air of such a height no warm-blooded creature 

 such as man could live. He used what the meteorologist 

 calls a ballon sonde, a small pilot balloon, to lift his instru- 

 ments. The results of these experiments corresponded well 

 with those obtained in Europe before the War, but the 

 source of the rays was still as obscure as ever. Though it 

 seemed fairly certain that their source was somewhere in 

 space, it did not appear probable that they came from any 

 special group of stars. One thing became very certain. 

 The newly discovered rays had nothing to do with the 

 sun, for they were just as powerful at midnight as at 

 midday. 



The next step was in the summer of 1923, when Millikan 

 and Otis took electroscopes to the top of the lofty Ameri- 

 can mountain known as Pike's Peak. These electroscopes 

 were shielded with heavy lead screens. At the same time 

 Kolhorster was working on Alpine glaciers, measuring 

 how far the rays would penetrate ice. Both investiga- 

 tions revealed that the rays were astonishingly ( hard ' 

 — that is, had very great penetrative power — but the 

 mystery of their origin remained. 



Many scientists of repute had no belief in these cosmic 

 rays. In 1925 Hoffman, the well-known German scientist, 

 declared his belief that the rays were not of cosmic origin, 

 while in America Swann was of the same opinion. 



Outside the world of Science few people had even heard 

 of these strange rays or knew about the arguments for 

 and against their origin, but Millikan, who now had Dr 



