MOUNT WILSON SOLAR OBSERVATORY. 201 



Mr. Babcock has devoted most of the year to investigations in connection 

 with the question of a general magnetic field on the sun and in nebulae. 

 The preliminary experimental work involved the design and use of instru- 

 ments for measuring magnetic fields of the order of lOO gausses. The first 

 measurements were made with a test-coil of known magnetic area in con- 

 junction with a ballistic galvanometer, standard condenser, and standard 

 cell. But a much better method was employed later, depending upon the 

 deflection in the field of a coil of known magnetic area traversed by a known 

 current and hung on a bifilar suspension. By this means an accuracy of 

 about 0.5 per cent was obtained in measuring a field of lOO gausses. After 

 adjusting the strength of the field by the use of this instrument to about 

 125 gausses, an electric spark was produced in it and photographs of the 

 spectrum were taken, with the aid of special polarizing apparatus, to de- 

 termine whether the Zeeman effect could be detected for such weak fields. 

 Measurements of these plates yielded results in good agreement with the 

 values calculated from the known displacements in a field of 17,500 gausses. 



For this work it was found necessary to have an assortment of special 

 preparations of mica for analyzing circularly and elliptically polarized light 

 of different wave-lengths. Accordingly, about six weeks were devoted to 

 the selection, mounting, baking, and testing of a stock of mica for this 

 purpose, including a set of five compound quarter-wave plates of about 45 

 sections each, covering most of the visible spectrum, a similar set having 

 only 5 or 6 sections each, a set of circular quarter and half wave plates, 

 some in two sections and others in one, and several special preparations of 

 half, quarter, and eighth wave mica. The preliminary testing, for the selec- 

 tion and classification of the mica, was made by a comparatively rough visual 

 method. A photographic method was used for measuring the relative 

 retardations for different wave-lengths of the finished specimens. The 

 method depends upon the production of large relative retardations — from 

 30 to 90 wave-lengths — by plates of quartz cut parallel to the optical axis, 

 which are inserted between crossed nicols. The addition of the mica to be 

 tested causes the interference bands to shift different amounts for different 

 wave-lengths, depending upon the retardation of the mica. In addition to 

 the mounted preparations, a stock of unmounted mica was selected and 

 classified. 



After this a number of tests were made on two of our gratings for polar- 

 izing effects. For plane-polarized incident light vibrating in different azi- 

 muths, the state of polarization of the reflected diffracted light was deter- 

 mined for spectra of different orders. 



During the absence of Mr. Scares in Europe Mr. Babcock spent much 

 time on the study of the solar photographs for the detection of a general 

 magnetic field, mainly in connection with the supervision and summarizing 

 of the measures. 



Mr. Babcock has also devoted three weeks to a further study of the fre- 

 quency curves of magnetic triplets, a month to the photography of the Zee- 



