COSMIC RAYS MILLIKAN AND CAMERON 215 



" penetratinii' rays." if they came from above^ were actually more 

 penetrating than had been supposed up to that time. Since if the 

 rays come in from above, the ionization inside airtight electroscopes 

 must increase exponentially — that is, geometrically with the distance 

 of rise toward the top of the atmosphere — these very high flights 

 were, and are now, especially significant. They place very certain 

 and very definite upper limits upon the absorption coefficients of the 

 rays entering the atmosphere, if there are in fact such rays. 



The fact, however, that the total discharge of the electroscopes in 

 these fliglits Avas but about one- fourth Avhat it should have been 

 from the absorption coefficients, computed on the cosmic ray hypothe- 

 sis from the data of Hess and of Kolhorster, suggested some other 

 cause for the phenomenon. For up to this time the increasing rate 

 of discharge with altitude was the chief if not the sole phenomenon 

 upon which the hypothesis of rays of cosmic origin rested. But 

 other alternatives were possible and had indeed been suggested; such, 

 for example, as radioactive particles of unknown origin spread 

 through the upper regions of the atmosphere. Such an alternative 

 could be tested definitely by making direct measurements of the 

 coefficients of absorption of the penetrating rays rather than at- 

 tempting to compute these coefficients as had heretofore been done, 

 on the assumption that the rays entered the atmosphere from above. 

 For if the rays were of radioactive origin, they would not be 

 expected to be appreciably harder than those of the known radio- 

 active substances such as thorium D or radium C. 



The next step was taken during summer of 1923, when Kolhorster ^^ 

 in Europe and Millikan and Otis ^- in America independently made 

 the first direct absorption measurements with materials other than 

 the atmosphere itself — the former in Alpine glaciers and in shallow 

 bodies of w^ater at sea level, the latter in thick lead screens carried 

 to the top of Pikes Peak— for the sake of throAving new light on the 

 possible origin of the penetrating rays. 



Kolhorster reported as a result of his glacier experiments an ab- 

 sorption coefficient of 0.25 per meter of water, or about half that 

 previously found, namely, 0.55, thus eliminating the discrepancy 

 between the findings from his balloon flights and our sounding bal- 

 loon experiments. He states in the paper describing this work that 

 his experiments prove definitely the existence of gamma rays of 

 about one-tenth the absorption coefficient of the hardest known 

 gamma rays (4.1 per meter of water), ^^ but speaks with reserve about 

 their origin. He says, after discussing various alternatives, that 

 " one inclines more and more of late to the view that tlie penetrating 



" Kolhorster, Sitz. Ber. d. Preuss. Akad., 34, 366, Dec. 20, 1923. 



i^" Millikan and Otis, I'liys. Rov., 23, 778; April, 1924. Also 28, S.'il ; 192G. 



" Radioactivity, Bull. Nat. Res. Council, Kovarik and McKeehan, p. 114, 1925. 



