DOCUMENT 483 NOVEMBER 1913 567 I fear there is no possibility of detecting the effect in full sunlight, for the following reasons: 1. The sky increases greatly in brightness near the sun, even under good observing conditions. I cannot now say at what distance bright stars would be visible, but will have observations made to determine this. 2. On Mount Wilson the best definition of the solar image is obtained only for about an hour in the early morning. Hence the atmospheric refrac- tion, changing rapidly with the hour angle, would be a troublesome obstacle. 3. It would be necessary to measure the differential change in distance of the star from the sun's limb, which would be difficult because of the low precision of micrometer settings on the limb, and the large distance (probably much beyond the range of an ordinary micrometer). The eclipse method, on the contrary, appears to be very promising, as it eliminates all of these difficulties, and the use of photography would allow a large number of stars to be measured. I therefore strongly recommend that plan. In a short time, as soon as some additional data are available, I wish to ask your opinion regarding the theory of the general solar magnetic field which I have recently detected by observation of the Zeeman effect. Believe me, with kind regards to Professor Maurer, Yours very sincerely, TLC (CSmH, George Ellery Hale Papers). Wright, Warnow, and Weiner 1972, pp. 68-69. [12 117]. Doc. 477. The astronomer William Wallace Campbell (1862-1938) was Director of the Lick Obser- vatory of the University of California on Mount Hamilton. Erwin Freundlich. Hale described Einstein's letter of 14 October in one to Campbell two weeks later in which he requested Campbell to "let him [Einstein] know what you think might be done at eclipses, as Dr. Eppstein [Paul Epstein] said you had this under consideration" (see George Hale to William Campbell, 1 November 1913, CPIT, Hale Papers, Box 9). Three days later Campbell responded that he "shall write to Professor Einstein concerning the eclipse problem of recording stars whose rays pass close to the sun, to test the gravitational deflection effect. We have undertaken to secure photographs of this kind for Dr. Freundlich of the Berlin Obser- vatory, using some of the lenses which we employed at past eclipses in search for intramercu- rial planets. I think the chances for success in this are good" (see William Campbell to George Hale, 4 November 1913, CPIT, Hale Papers, Box 9). The solar observatory of the Carnegie Institution of Washington, located in the hills above the California Institute of Technology in Pasadena. See Hale 1913. Julius Maurer.