MOUNT WILSON SOLAR OBSERVATORY. 255 



resting on four steel balls, and moved as a whole by an electric motor. 

 Thus the solar image and photographic plate are stationary, while the 

 collimator and camera slits move with the spectrohehograph across 

 them. The 4-inch plane grating used in this instrument, which is 

 extremely bright in the first order, was ruled by Anderson. As the 

 axes of the collimator and camera lenses are parallel, the beam, after 

 leaving the grating, falls on a plane mirror, from which it is reflected 

 through the camera lens. This mirror may be moved along the axis 

 of the camera lens, thus making it possible to change the angle between 

 the incident and diffracted beams and hence to vary the linear disper- 

 sion of the first-order spectrum. 



The two-slit device giving images of sun-spots and flocculi simul- 

 taneously on a single plate for differential measurement has been 

 mentioned above (p. 254). By providing two gratings or two mirrors 

 below the camera lens, two images of the spectrum from a single colli- 

 mator sht can be produced side by side. Thus by setting one camera 

 sUt on the red edge of Ha in one spectrum and a second camera sUt 

 on the violet edge of Ha in the other spectrum, photographs of flocculi 

 can be obtained with the two edges simultaneously. Similarly two or 

 even three photographs, showing sections of the floccuU at as many 

 different levels, can also be taken simultaneously. 



Other improvements in solar instruments include an arrangement by 

 which a single prism, for photography with K2, can be quickly substi- 

 tuted in the 5-foot spectrohehograph for the two prisms used with 

 Ha] a new set of electric hand controls and cables for the quick and 

 slow motions of the 75-foot spectrograph and 150-foot tower telescope; 

 and a new quadruplex electric pump for the water-circulating system 

 of the ccelostat mirrors of the latter instrument, which eliminates the 

 vibration produced by the old pump. 



SOLAR PHOTOGRAPHY. 



During the year, 284 direct photographs of the sun have been taken 

 with the Snow telescope, and the daily work with the 5-foot spectro- 

 hehograph has been continued by Messrs. Ellerman, Colby, Monk, 

 Capon, Luckey, and Campbell. Of the total number of 1,072 negatives 

 secured, 507 are Ha disk plates, 293 are K2 disk plates, and 272 are K2 

 prominence plates. When interesting floccuh were present, series of 

 successive exposures were also made with Ha. 



The 13-foot spectrohehograph of the 60-foot tower telescope was 

 installed in July and first used on July 25. Since that date 225 nega- 

 tives of interesting floccuh and prominences, each containing from 1 

 to 12 exposures, have been obtained with it by Messrs. Ellerman, 

 Colby, Monk, and Luckey. These include daily photographs (since 

 August 3) of a 2-inch solar image with the center of Ha and series of 

 photographs of interesting regions of a 6.5-inch solar image. Special 



