MOUNT WILSON SOLAR OBSERVATORY. 257 



the grating supports also the interferometer and the mirrors required to 

 project the interference pattern upon the sht. The whole apparatus is 

 effectively protected by a cover of wood and building paper. For pro- 

 jecting light upon the interferometer, a concave mirror of 8 inches 

 diameter and 2 feet focal length has been provided, thus making an 

 entirely achromatic system. This mirror and the one of longer focus 

 referred to above were constructed in the optical shop by Mr. Kinney. 

 The camera accommodates plates 2.5 by 12 inches. Curvature of the 

 field reduces the effective aperture to 8 inches in the center for flat 

 plates, but this permits a range of 1000 a to be photographed at one 

 exposure in the first order, in which the scale is very nearly 5 a per 

 millimeter. For wave-lengths less than X 4000 the second order is 

 generally used, on account of the increased dispersion. Wave-length 

 X 9000 can be reached in the first order. 



The vacuum arc has been modified and is now in use for producing 

 the primary standard of wave-length — the red cadmium line. It 

 may readily be used for an iron arc when desired. 



An adapter for a measuring machine has been constructed, by means 

 of which two rectangular coordinates may be measured without dis- 

 turbing the photographic plate. For the reduction of interferometer 

 plates covering a wide range of spectrum, this saves the continual dis- 

 placement and readjustment of the plate which is otherwise necessary, 

 and also facilitates the identification of the lines. Two more of these 

 attachments are now under construction in the instrument shop. 



A small ccelostat has been built for conveniently supplying a beam of 

 sunlight for laboratory purposes. 



Our stock of interference apparatus has been increased by the addi- 

 tion of an invar etalon of 15 mm. length, adjusted to about one-tenth of 

 a wave-length. 



Pole pieces of ferro-cobalt have been provided for the large magnet. 

 For the excitation and air gap cominonly used, they increase the field- 

 strength about 4 per cent. 



A rotary high-vacuum oil-pump has been purchased and installed for 

 the study of the Stark effect. 



ELECTRIC-FURNACE SPECTRA. 



Mr. King's experimental work with the electric furnace has dealt 

 with the following problems: 



Variation of Metallic Spectra with Temperature. 



The furnace spectra of calcium, strontium, barium, and magnesium 

 have been studied by the method already employed for several other 

 elements, the spectra given by furnace temperatures ranging from 

 1650° to 2500° C. being photographed for the range X 2600 to X 7200. 

 Approximately 500 lines are included in this range for the four elements. 



