"High Frequency Rays of Cosmic Origin" I, II, and III, 3 papers in Physical Review, 1926. **Determining that the Origin of Cosmic Rays is from Outside the Solar System.

**Determining that the Origin of Cosmic Rays is from Outside the Solar System (1926)** The three papers by Robert A. Millikan (with I.S. Bow, R.M. Otis, and G.H. Cameron) in three issues of the Physical Review, April, June, and November, 1926: R.A. Millikan and I.S. Bow. "High Frequency Rays of Cosmic Origin, I: Sounding Balloon Observations at Extreme Altitudes", in Physical Review, April, 1926, volume 27 #4, pp 353-362 in the issue. A GOOD+ copy, with about an inch of spine cover missing at the top with no loss of text. R.A. Millikan and R.M. Otis. "High Frequency Rays of Cosmic Origin, II: Mountain Peak and Airplane Observations", in Physical Review, June, 1926, volume 27 #6, pp 645-659 in the issue. Original wrappers. There's a little chipping at the top of the spine cover, with a tiny bit of loss of text to the first word on the spine. GOOD+-VG R.A. Millikan, G.H. Cameron. "High Frequency Rays of Cosmic Origin III. Measurement in Snow-fed Lakes at High Altitudes", in Physical Review, November 1926, volume 28, #5, pp 851-869 in the monthly issue. Original wrappers. GOOD copy (if not better) though there is a 1-inch tear at spine cover top. Scarce in the original wrappers.…