50 



NATURE 



[November ii, 1909 



SOLAR VORTICES AND MAGNETIC FIELDS. 



II. 

 T HAVE already referred to the importance of applying 

 "*• in astronomical research the methods of the physicist. 

 During the last quarter of a century the study ot spectro- 

 scopic phenomena in the laboratory has been completely 

 transformed. It may well be said that this transformation, 

 which has involved such discoveries as spectral series, the 

 effect of pressure on wave-length, and the Zeeman effect, 

 has been directly due to the use of Rowland's concave 

 gratings, of great focal length, arranged for photography. 

 In astronomical spectroscopy great advances have also been 

 made, but the spectroscope has continued to occupy the 

 place it formerly held as an attachment of the telescope. 

 .Although Rowland used a long-focus concave grating for 

 his classic study of the solar spectrum, the heliostat and 

 Jens employed with this instrument gave so small a solar 

 image on the slit that the investigation of sun-spots and 

 other details was impossible. We thus see that while in 

 the observatory the spectroscope continued to be used as 

 an accessory of the telescope, in the laboratory the parts 

 Avere exchanged and the telescope was employed simply as 

 an accessory of the spectroscope. It seemed obvious that a 

 great opportunity for advance lay open to the investigator 

 who would combine a long-focus 

 spectroscope with a long-focus tele- 

 scope, .^s it would be difficult, or 

 perhaps impossible, to use for photo- 

 graphy a sufficiently long spectroscope 

 attached to the tube of an equatorially 

 mounted telescope, some form of fixed 

 telescope was plainly essential. 



The tower telescope on Mount 

 Wilson (Fig. 5) is designed to accom- 

 plish this purpose. It consists essenti- 

 ally of a 12-inch refracting telescope, 

 of 60-feet focal length, mounted in a 

 fixed position, pointed directly at the 

 zenith. The ordinary telescope tube is 

 replaced in this case by a light steel 

 tower, firmly held in position by steel 

 guy ropes. The 12-inch objective lies 

 'horizontally at the summit of the 

 tower, and sunlight is reflected into it 

 from the second of two adjuslablr 

 plane mirrors. The first of these 

 mirrors is mounted as a ccelostat, and 

 is rotated by an accurate driving- 

 clock about a polar axis at such a 

 rate as to counteract the apparent 

 motion of the sun. Thus a beam of 

 sunlight is reflected from the coelostat 

 mirror to the second mirror, which 

 sends it vertically downward through 

 the objective. In the focal plane, 

 60 feet below the objective, an image 

 ■of the sun, about 66 inches in 



diameter, is formed on the slit of a spectrograph, at a 

 height of about 3 feet above the surface of the ground. 

 -After passing through the slit, the light of any desired 

 portion of the solar image (a sun-spot, for example) 

 descends vertically into a well about 30 feet deep 

 excavated in the earth beneath the tower. Thirty feet 

 ■ from the slit the diverging rays encounter a 6-inch objec- 

 tive, through which they pass. After being rendered 

 parallel by the objective, the rays fall upon a Rowland 

 plane grating, ruled with 14.438 lines to the inch. The 

 grating breaks up the light into a series of spectra, and 

 the rays are returned through the same objective, which 

 brings the spectra to a focus at a point near the slit. By 

 inclining the grating at a small angle, the image of the 

 spectrum is made to fall at a point slightly to one side of 

 the slit, and here the photographic plate is placed. Thus 

 a portion of the spectrum 17 inches in length can be 

 photographed in a single operation. In the work on sun- 

 spots, most of the photographs are taken in the third 

 order of the grating, where the dispersion and resolving 

 power are very high. When the spot spectrum is beinc 

 photographed, only the light from the umbra is admitted 



' Discourse delivered at ihe Rov.-il Instituti.ni on Friday, May 14, by 

 Prof. George E. Hale, For.Mem.R.S. Continued from p. 23. 



£0 the slit. At the end of the exposure this portion of the 

 slit is covered, and light from the photosphere, at a point 

 removed from the spot, is admitted to the slit on either 

 side. Thus the narrow spot spectrum is photographed 

 between two strips of solar spectrum, used for comparison. 



The advantages of this combined form of telescope and 

 spectrograph are considerable. On account of the. great 

 thickness (12 inches) of the inirrors, the height of the 

 coelostat above the heated earth, and the use of a vertical 

 beam, the definition of the solar image is always better 

 than with the Snow (horizontal) telescope. Another 

 important advantage is the nearly constant temperature 

 at the bottom of the well, where the grating is placed. 

 This perinits long exposures to be given, when necessary, 

 without danger of such displacements of the spectral lines 

 as would be caused by expansion or contraction of the 

 grating. The grating used in this spectrograph is a small 

 one, which I have einployed in most of my work since 

 1889, but the unusual focal length of the spectrograph 

 permits the full visual resolution of the grating to be 

 utilised in photographic observations. Thus it lias become 

 possible to photograph the widened lines and doublets, as 

 well as a host of narrow lines, most of them due to 

 chemical compounds, which had not previously been 

 recorded in the spot spectrum. 



NO. 2089, VOL. 82] 



(i. 5. — Tower Telescope 



Lack of time prevents me from discussing in this lecture 

 the various studies of sun-spot lines carried out with this 

 instrument before the attempt to detect a magnetic field in 

 spots was undertaken. .An extensive catalogue of these 

 lines is nearly complete, a preliminary map has been issued 

 and a better one is, in preparation, and a series of investi- 

 gations with the arc and electric furnace has suggested 

 that the strengthening and weakening of certain lines is 

 due to a reduction in the temperature of the spot vapours. 

 .At present we are concerned with the cause of the widen- 

 ing and doubling of spot lines, and the method of testing 

 this question must now be described. 



-A Nicol prism was mounted above the slit of the spectro- 

 graph, and just above this a Fresnel rhomb. If the com- 

 ponents of a spot doublet were circularly polarised in 

 opposite directions, passage through the rhomb should give 

 two plane polarised beams, the planes of polarisation 

 making an angle of 90° with each other. Thus in one 

 position of the Nicol one of the components should be 

 photographed alone, and by turning the Nicol 90° this 

 should disappear and the other component come into view. 



When this test was applied with the tower telescope, in 

 June, icioS. the true character of the spot doublets became 

 apparent (Fig. 6). One or the other coinponent of the 



