MOUNT WILSON SOLAR OBSERVATORY. 1 79 



quirements of each class of work. The ease with which the diameter of 

 the image can be changed, by swinging into place objectives of 30, 60, and 

 150 feet focal length, and the equal facility of changing the dispersion by a 

 device similar to the multiple nose-piece of a microscope, renders possible 

 a great variety of work in a limited time. 



The combined spectrograph and spectroheliograph is also designed for the 

 photography of the spectrum of the chromosphere ("flash spectrum"), the 

 comparative study of the spectra of center and limb, the measurement of 

 the solar rotation, the determination of the pressure at different levels in the 

 solar atmosphere, and for various other purposes. 



The auxiliary apparatus includes a reflecting slit, especially adapted for 

 the photography of the spectra of pores and other minute regions ; a device 

 for securing uniform density of the spectra of umbra, penumbra, and pho- 

 tosphere in a single exposure ; a special guiding device, to maintain any part 

 of the sun's image on the slit with great accuracy throughout an exposure; 

 a parallel-motion apparatus, to facilitate accurate orientation of the instru- 

 ment; polarizing apparatus, with simple and compound quarter and half 

 wave plates for various wave-lengths; and an electric arc for comparison 

 spectra. 



General Magnetic Field of the Sun. 



In the absence of sun-spots and other local disturbances, which might tend 

 to obscure the general phenomenon, an attempt has been made to observe 

 any slight Zeeman effect due to the weak magnetic field of the sun. 



Assuming that a moving molecule carries a magnetic field with it, Schuster 

 has calculated that the magnetic intensity of the sun should be about 440 

 times greater than that of the earth, if both are due to the effect of rotation.* 

 With similar considerations in mind, the Director attempted in 1908 to detect 

 the sun's field with the 60-foot tower telescope. The extremely small line 

 displacements observed at that time could not be safely attributed to the 

 general field, as it was necessary to rotate the Nicol prism between exposures, 

 and the unavoidable change in the illumination of the grating might easily 

 result in spurious shifts. A compound quarter-wave plate, consisting of a 

 number of mica strips 2 mm. wide, the principal sections of successive strips 

 making an angle of 45 ° with the axis of the nicol and 90 with each other, 

 was accordingly ordered from Werlein. Thanks are due M. Cotton for val- 

 uable advice and assistance in the supervision and testing of this and other 

 polarizing apparatus obtained from the same maker. An investigation was 

 also undertaken of the polarization phenomena caused by the mirrors of the 

 tower telescope, which might lead to erroneous results unless measured and 

 corrected. The special polarimeter built for this purpose permits the polar- 

 ization corresponding to any position of the sun to be measured in a few 

 moments, either for the 60-foot tower or for the different arrangement of 

 the mirrors in the 150- foot tower. 



* Proc. Phys. Soc. London, vol. 24, part 3, p. 127, April 15, 1912. 



