November 4, 1909] 



NATURE 



23 



corpuscles at high temperature is common to carbon and polarised in planes at right angles to one another. A 



to metals, whether in the solid or in the vaporous con- .Nicol prism, standing at a certain angle, will transmit one 



dition. Thus we have warrant for the belief that the sun, of these plane polarised beams and cut off the other, 



composed of just such elements as constitute the earth. Turning the Nicol through 90° will cause the component 



must emit great numbers of these corpuscles. .As Thomson I previously cut off to be transmitted, and the other to be 



has estimated that the rate of emission of a carbon filament I stopped. 



at its highest point of incandescence may amount to a i Consider a sun-spot at the centre of the solar disc, and 



current equal to several amperes per square centimetre of i suppose it to be produced by a vortex, the axis of which 



surface, we can hardly be mistaken in assuming the exist- I lies on the line passing from the eye of the observer 



ence of still more powerful currents in the sun. The 

 emission of negatively charged particles implies the emission 

 of positively charged particles, but in laboratory experi- 

 ments, because of unequal rates of diffusion or other causes, 

 charges of one sign are always found to be in excess. We 

 thus have reason to believe that powerful magnetic fields 

 may result from the revolution of these particles in the 

 solar vortices. 



In seeking a means of detecting such fields, let us first 

 recall Faraday's discovery of the effect of magnetism on 

 light, made at the Royal Institution in 1S46. This dis- 

 covery relates to the rotation of the plane of polarisation 

 of light when pjtssed through a plate of dense glass in a 

 strong magnetic field. .Although Faraday, in what was 

 said to be his last experiment, endeavoured to detect the 



. 4. — Sun-spots and Hydrogen FloccuH, showing Right- and Left-handed Vort 

 1908, October 7, 7h. ozm. a.m. P.S.T. 



effect of magnetism on the lines of the spectrum, he failed 

 because the apparatus then available was not sufficiently 

 powerful. In 1896 Prof. Zeeman examined with a large 

 spectroscope the two yellow lines emitted by sodium vapour 

 in a flame between the poles of a powerful magnet. 

 Observing in the direction of the lines of force, he saw 

 that the sodium lines widened when the magnet was 

 e.xcited. .Subsequently, with more powerful apparatus, he 

 found that a single line, when observed -under the above 

 conditions, is split into two corr.ponents by a magnetic 

 field. The distance between the two components is a 

 measure of the strength of the field : but the most 

 characteristic quality of these double lines, which dis- 

 tinguishes them from double lines produced by any other 

 known means, is the fact that the light of the two com- 

 ponents is circularly polarised in opposite directions. If, 

 then, we encounter a double line in the spectrum of any 

 substance, and suspect it to be due to a magnetic field, we 

 must apply the test for circular polarisation. 



The simplest means of testing for circularlv polarised 

 light is to transform it into plane polarised light by pass- 

 ing it through a quarter-wave olate or a Fresnel rhomb. 

 Tn the case of a Zeeman doublet, we would then have 

 issuing from the rhomb the light of the two components, 

 NO. 2088, VOL. 82] 



through the spot to the centre of the sun. In these circum- 

 stances, if a strong magnetic field is produced by the 

 vortex, the spectral lines due to vapours lying within this 

 field should be widened or transformed into doublets. 

 Moreover, the light of the components of these doublets 

 should be circularly polarised in opposite directions. This 

 would be true if the spot vapours were emitting bright 

 lines, identical in character with those emitted by a 

 radiating vapour between the poles of a magnet. The 

 experiments of Zeeman, Cotton, Kiinig, and others, show, 

 however, that dark lines, produced by the absorption of 

 the spot vapours, should behave precisely in the same way 

 as bright lines. 



The spectrum of a sun-spot was observed for the first 

 time by Lockyer in 1866. He found that many of the 

 lines of the solar spectrum were widened 

 where they crossed the spot, and the 

 observation of these widened lines has been 

 carried on systematically by many observers 

 ever since. Conspicuous among these 

 observers was Young, whose last observa- 

 tions were made with a powerful grating 

 spectroscope attached to the 23-inch Prince- 

 ton refractor. This instrument showed 

 that some of the spot lines are close 

 doublets. Dr. Walter M. Mitchell, who at 

 first worked in conjunction with Prof. 

 Young, and later by himself, gave special 

 attention to these double lines, which he 

 found to be particularly numerous at the 

 red end of the spectrum. He called them 

 " reversals," and the existing evidence 

 Livoured the view that they were produced 

 by the radiation of a hotter layer of 

 vapours overlying the spot, which would 

 ^ive rise to a narrow bright line at the 

 centre of the widened dark line. True re- 

 versals of this kind actually seem to occur 

 in the case of H and K and other lines in 

 the spot spectrum, and it was therefore 

 natural that Mitchell should attribute the 

 similar phenomena of the spot doublets to 

 a similar cause. It was generally sup- 

 posed that the widening of the dark lines 

 was due to the increased density of the 

 sp,ot vapours. The diverse character of 

 the lines in the sun-spot spectrum is well 

 illustrated by this drawing, which is due 

 to Mitchell. In addition to the ordinarv 

 widened and " reversed " lines we find cases where a dark 

 central line is accompanied by wings, others in which lines 

 are thinned or completely obHterated, &c. 

 (7'o he continued.) 



RECENT AGRICULTURAL PUBLICATIONS 

 FROM THE WEST INDIES. 



THE imperial Department of Agriculture for the West 

 Indies issue three periodical publications : — (i) the 

 West Indian Bulletin, a quarterly scientific journal con- 

 taining papers or reports by members of the scientific 

 staff, or papers read at agricultural conferences; (2) the 

 Agricultural News, a fortnightly paper, published at one 

 pennv only, containing short articles in popular language 

 on subjects of importance to planters ; (3) a series of 

 bulletins, each containing detailed information on some 

 special subject. In addition, reports are issued on the 

 work done at the different experiment stations on some 

 of the islands, and the Department^ of Agriculture for 

 Jamaica issues a .separate bulletin of its own. 



During the present vear the last number of vol. ix. of 

 the West Indian Bulletin and the first of vol. x. have 

 been published. The timbers of Jamaica are described at 



