D O C U M E N T 4 2 5 D E C E M B E R 1 9 2 6 6 5 3 the aid of new photographic sensitizers now available. Finally, I think we can elim- inate personal equation by measuring the plates with the aid of an improved Koch recording micro-photometer. We are now entering the rainy season, and on this ac- count, and also, because of the present solar activity, a long time may elapse before any new results can be obtained. It is hardly necessary to say that I am greatly interested in your new theory of gravitation and magnetism and anxious to learn more about it. I am especially cu- rious to know whether it will account for the rapid decrease of field-strength which we found at increasing elevations in the solar atmosphere. This rapid decrease, so different from the slow decrease with altitude observed in the case of a uniformly magnetized sphere, has always puzzled me greatly. It raises the question, of course, whether the maximum field-strength observed by us may not be surpassed at lower levels than we could reach in the solar atmosphere. Any suggestions with which you may favor me will be gratefully received, as I am anxious to make our results as useful as possible to you. As you doubtless know, Professor Michelson[5] will soon repeat the Michelson- Morley experiment on Mount Wilson with an improved interferometer. In my opin- ion he is not likely to confirm the strange results of Professor Miller.[6] Professor Millikan[7] has probably written you about the purely negative results recently ob- tained by Dr. Kennedy[8] —in Pasadena and on Mount Wilson. We expect Professor Lorentz here in January[9] and he can doubtless tell us something of your recent work. We were greatly disappointed that your health did not permit you to visit Pasadena with him.[10] May we not hope that you can come in the near future for a stay of several months? Believe me, with kind regards Yours very sincerely. George E. Hale P. S. I hope we can successfully remeasure some of our old plates with the Koch microphotometer, which has been considerably improved since the test of Dr. van Maanen’s measures described in our second paper on the general magnetic field, p. 6 (Mount Wilson Contributions, No 148,[11] sent under separate cover). You will find other discussions of various possible sources of error and their elimination in this paper and the earlier one.[12] I will talk over the whole question again with Pro- fessor Seares.[13] G. E. H. TLS. [12 073]. Written on letterhead “Carnegie Institution of Washington Mount Wilson Observatory Pasadena, California.”