COSMIC RAYS MILLIKAN AND CAMERON 223 



tofore known, nameh', those accompanying radioactive processes. 

 Otherwise stated, these experiments show conclusively that 

 there are no radiations of appreciable intensity entering the 

 earth's atmosphere having frequencies intermediate between those 

 of the gamma rays and those of the cosmic rays. For, since the 

 hardest gamma rays are capable of penetrating a thickness of about 

 70 centimeters of water, while, as indicated beloAV, the cosmic rays 

 are capable of penetrating 70 meters of water, and since penetrating 

 power increases approximately as frequency, if rays of appreciable 

 intensity came into the atmosphere having frequencies between 

 those of gamma rays and those of the cosmic rays, they would of 

 necessity have caused the rapid discharge of an electroscope which 

 rose to within the equivalent of 80 centimeters of water of the top 

 of the atmosphere, the whole of the earth's atmosphere being the 

 equivalent of 10 meters of water. No such rapid discharge took 

 place; hence there are no strong radiations entering the earth's 

 atmosphere in that particular region of frequencies. 



Second. — The experiments of Millikan and Cameron of the sum- 

 mer and fall of 1927 made in deep, high-altitude California lakes 

 I with new electroscopes eight times more sensitive than those the 

 \ authors had theretofore used, brought to light the definite proof 

 I that the cosmic-ray spectrum consists of definite bands, like those 

 of neon or mercury lamps, containing spectral lines as much as three 

 octaves apart, the highest frequency band having so enormous a pene- 

 I trating power that it passes through as much as 200 feet of water 

 (18 feet of lead) before becoming completely absorbed. This dis- 

 l covery of a banded structure in the cosmic rays shows that they are 

 [i not produced as are X rays by the impact upon the atoms of matter 

 if of electrons which have acquired large velocities by falling through 

 powerful electrical fields, as we had earlier suggested ; but that tJiey 

 are leather produced hy definite and continually recurring atomic 

 trans forTiuitions involving very much greater energy changes than 

 \: any occurring in radioactive processes. The proof of the banded 

 I structure of the cosmic-ray spectrum is found in the fact that the 

 j: new ionization-depth curve (fig. 1), which is seen to be of very 

 i: much higher resolving power than any heretofore obtained, when 

 Pi taken in connection with the sounding balloon data, shows a nearly 

 p constant absorption coefficient from close to the top of the atmos- 

 ! phere down to about sea level. 10 meters below the top, then bends 

 :i quite suddenly and uncovers below 15 meters a new absorption co- 

 I, efficient of only about one-sixth the former value, which continues 

 ■ down to 70 meters below the top with but little further change. 

 I The cosmic rays therefore consist of at least two, possibly more (see 

 below), radiation bands of absorption coefficients (and therefore, 

 , roughly, also of frequencies) in the ratio of 8 to 1. The sharpness 



