MOUNT WILSON OBSERVATORY. 251 



third order, a 4-incli plate covers a little less than 400 a, corresponding 

 to an average dispersion of about 4 a per milUmeter. 



Our knowledge of the Zeeman effect has been obtained chiefly 

 through a study of spark spectra, produced between the pole-tips of 

 large electro-magnets, where the uniform field is so limited that only 

 small sources, such as sparks or vacuum tubes, can be used. It is 

 becoming more and more necessary to study the spectra of other 

 sources in a strong magnetic field, especially that of the electric furnace. 

 This requires a fairly uniform magnetic field of high strength through- 

 out a space at least 2 inches in diameter by 4 inches in length. Mr. 

 Anderson's calculations show that for a water-cooled solenoid ^^dth- 

 out any iron approximately 400 k. w. are required to give a field of 

 45,000 gausses in such a space, and that for fields of this size and 

 intensity there is little or no advantage in the use of iron. A 500 k.w. 

 motor-generator set has accordingly been ordered, to furnish about 

 4,000 amperes direct current at 125 volts. An underground tank hav- 

 ing a capacity of about 5,000 cubic feet will be constructed to hold 

 the water necessary for cooling purposes. Solenoids of various sizes to 

 suit different purposes wdll be constructed as needed, as the amount 

 of copper required for a single solenoid is very small, ranging from 

 10 to 50 pounds. When this apparatus is installed the laboratory 

 will have exceptional facilities for the study of the Zeeman effect, so 

 necessary in the interpretation of sun-spot spectra. It is also hoped 

 that the study of arc spectra with very heavy currents may lead to 

 interesting results. 



In order to make the interferometer spectrograph available for 

 observations in the ultra-violet, three new speculum mirrors have 

 been ground and figured in the optical shop. One of these, 6 inches 

 in diameter and 21 feet radius of curvature, is used as a collimator 

 in the auxiliary spectrograph; a 2-inch mirror of 50 inches radius of 

 curvature is used for projecting the interference pattern upon the 

 spectrograph slit, while the third mirror, 6 inches diameter and 48 

 inches radius of curvature, is for projecting an image of the source 

 upon the etalon. 



A new film-holder receiving a film about 16 inches long, has 

 been made in the shop for use -with the interferometer spectrograph. 

 It is adjusted to the focal curve of the grating so as to bring 2,000 a 

 into good focus in the first-order spectrum. This permits a much 

 greater number of standards to be observed upon each photograph, 

 thus increasing the accuracy attainable in the relative values of the 

 wave-lengths. 



ELECTRIC-FURNACE INVESTIGATIONS. 



The following lines of work with the electric furnace were pursued 

 during the year by Mr. King, in part with the regular tube-furnace 

 and in part with the special form constructed for use with the Weiss 

 electro-magnet. 



