The shortest wave-lengths we have so far had under discussion are those of the γ-rays, but the last line of the table refers to radiation with a wave-length of only about a four-hundredth part of that of the shortest of γ-rays. Since 1902, various investigators, Rutherford, Cooke, McLennan, Burton, Kolhörster and Millikan in particular, have found that the earth’s atmosphere is continually being traversed by radiation which has enormously higher penetrating power than any known γ-rays. By sending up balloons to great heights, Hess, Kolhörster, and later Millikan and Bowen, have shewn that the radiation is noticeably more intense at great heights, thus proving that it comes into the earth’s atmosphere from outside. If the radiation had its origin in the sun and stars, the main part of the radiation received on earth would come from the sun, and the radiation would be more intense by day than by night. This is found not to be the case, so that the radiation cannot come from the stars, and so must originate in nebulae or cosmic masses other than stars. Millikan is confident that its sources lie outside the galactic system. _The Mechanical Effects of Radiation_ +------------+-----------+----------------+-------------+-----------+ |Wave-lengths| Nature of | Effect on Atom | Temperature | Where | | (cms.) | Radiation | | (deg. abs.) | found | +------------+-----------+----------------+-------------+-----------+ |7500 × 10⁻⁸ | Visible | Disturbs | 3,850°| Stellar | | to | light | outermost | to |atmospheres| |3750 × 10⁻⁸ | | electrons | 7,700°| | | | | | | | | 250 × 10⁻⁸ | X-rays | Disturbs inner | 115,000°| Stellar | | to | | electrons | to | interiors | | 10⁻⁸ | | | 29,000,000°| | | | | | | | | 5 × 10⁻⁹ | Soft | Strip off all | 58,000,000°| Central | | to | γ-rays | or nearly all | to |regions of | | 10⁻⁹ | | electrons | 290,000,000°|dense stars| | | | | | | | 4 × 10⁻¹⁰| γ-rays of |Disturbs nuclear| 720,000,000°| ? | | | radium-B | arrangement | | | | | | | | | | 5 × 10⁻¹¹| Shortest | —— | 5.8 × 10⁹ °| | | | γ-rays | | | | | | | | | | | 1·3 × 10⁻¹²| Highly |Annihilation or | 2.2 × 10¹² °| | | |penetrating| creation of | | |