6 5 2 D O C U M E N T 4 2 5 D E C E M B E R 1 9 2 6 425. From George E. Hale [Pasadena,] December 2, 1926 Dear Professor Einstein: Dr. Baade of the Hamburg Observatory,[1] who is spending some time here as an International Research Fellow, tells me that you have developed a new theory which may account for the general magnetic fields of the earth and sun.[2] He said you wished to know how closely our measures of the strength of the sun’s general field can be relied upon, and I am therefore sending you some comments on this question. As you know from our papers, the displacements caused by the Zeeman effect are so small, in the case of the sun’s general field, that I did not publish my first results until we had completed a large number of tests. Every device we could think of to eliminate personal prejudice on the part of the measurer was employed. Thus he never knew whether the plate he was measuring represented the northern or southern hemisphere of the sun, nor did he know the latitude. As a further precau- tion, the compound quarter-wave plate, used with a Nicol prism over the slit, was often inverted, thus reversing the sign of the displacement corresponding to any point of observation. Of course the measurer was left in ignorance of the position of this quarter-wave plate. The most consistent measures were those made by Miss Lasby[3] and Dr. van Maanen.[4] These agreed well in sign, but differed considerably in magnitude. The personal equation of the measurer may thus prove to be an important factor. Dr. van Maanen acquired a great skill in measurement, and his values of the field-strength corresponding to different lines may be reliable within ten or twenty percent. But the question of absolute values is a very different one, and I have long intended to make another study of the problem, using the 75-foot spectrograph of my solar lab- oratory in Pasadena, a new grating, very bright in the second order, and an optical system differing in some other respects from that employed on Mount Wilson. A complete redetermination of the sun’s general field cannot be made until the next sun-spot minimum, because of the disturbing effect of the intense magnetic fields of the numerous sun-spots now present. But intervals of small activity some- times occur, and with the aid of my new spectrohelioscope I can select regions on the sun free not only from spots but also from any disturbed condition of the solar atmosphere. I hope to get large Zeeman displacements, and at the same time to avoid any effect of scattered sun-light, by working well out in the infra-red, with
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