May 4, 1905] 



NA TURE 



RECENT SPECTROHELIOGRAPH RESULTS. 



T N a previous number of this Journal (vol. Ixix. 

 J- p. 609, 1904), under the heading of " A New 

 Epoch in Solar Physics," 1 gave an account of the 

 magnificent work that Prof. Hale had recently been 

 accomplishing at the Yerkes Observatory with his 

 latest form of spectroheliograph, the instrument bemg 

 worked in conjunction with the great 42-inch Yerkes 

 refractor, which forms an image of the sun seven 

 inches in diameter. 



In the present article it is proposed to give a brief 

 description of another instrument based on the same 

 principle, an account of which was published by M. 

 Janssen, and to indicate some of the results which 

 have been obtained with it. This instrument has 

 been at work at the Solar Physics Observatory durmg 

 the past year, and in a recent comnninir.Ttion to the 



which the solar image is moved across the primary 

 slit by means of the declination motor which moves 

 at the same time and rate the photographic plate; 

 or the primary slit, and with it the whole spectro- 

 heliograph, may be moved across the image formed 

 at the focus of the equatorial. The first method is 

 that adopted at the Yerkes Observatory, and the 

 second that at Potsdam. 



There is a further method in which a stationary 

 solar image is formed by means of a siderostat and 

 lens, and the spectroheliograph is mounted hori- 

 zontally and moved in an east and west direction 

 across this fixed image. Such a mode of procedure 

 is that employed at South Kensington. 



The advantage of the last mentioned arrangement 

 is that there is no limit to the size or weight of the 

 spectroheliograph ; the uniform motion required can 

 be easilv and efficiently secured, and lastly, this 



Royal Astronomical Society I gave a more full 

 account of it, to which reference can be made for more 

 detailed information than is here given. 



It is not necessary in this place to refer at any length 

 to the principle which underlies the construction of a 

 spectroheliograph, since this was referred to m the 

 article above mentioned. It will suffice here to say, 

 therefore, that the pictures produced by this new 

 method of solar research give us photographs of the 

 sun in monochromatic light, or in rays of any par- 

 ticular wave-length that is desired. Thus if we re- 

 quire to study "the distribution of hydrogen on or 

 around the solar disc we employ a line in the spectrum 

 of hydrogen, if calcium a calcium line, or iron an 

 iron line. 



There are, however, several methods of using the 

 spectroheliograph. This instrument may either be 

 employed in conjunction with a large equatorial, in 



NO. 1853, VOL. 72] 



motion does not in any way affect the steadiness of 

 the solar image under examination. . 



The South Kensington instrument was erected in 

 the year 1903, but it was not until last year that satis- 

 factory photographs were secured and routine work 

 begun. This success was due to the use of a larger 

 lens {12-inch) for throwing the solar image on the 

 primary slit, the previous lens of 6 inches aperture 

 not giving a sufficiently bright image. 



In this curtailed description of the instrument refer- 

 ence of anv length need onlv be made to the spectro- 

 heliograph' proper. There is nothing particularly 

 novel about the siderostat, except, perhaps, its more 

 than usual size, the large mirror of 18 inches 

 diameter, the two small motors for operating the slow 

 motions in right ascension and declination, and a 

 modified forni of Russell control for regulating 

 the speed of the driving clock. This instrument is 



