NATURE 



[August 5, 1897 



THE APPROACHING TOTAL ECLIPSE OF 



THE SUN> 



III. 



The Work proposed for the Indian Eclipse. 



A MONG the work proposed to be carried on during 



■'*' the eclipse of 1896, it may be well imagined that 



the employment of the prismatic camera occupied a 



large place, but, unfortunately, the weather allowed no 



observation to be made by those of 6 inches and 9 inches 



aperture I took out to Kio ; the instrument employed in 



Brazil in 1893 was, however, again successful in the hands 



of Mr. Shackleton in Novaya Zemlya. 



With regard to the work I propose to attempt m India, 

 the following extracts from a letter I was called upon 

 to write some time ago, still express my views. These 

 are based upon the results obtained with the prismatic 

 cameras in 1893 and 1896, which, although they are 

 not yet fully worked out, in my opinion far transcend in 

 importance any observations made on the eclipsed sun 

 since 1868 : — 



" (i) I propose in 1898 to use a prismatic camera with 

 double the present dispersion, although the dispersion employed 

 by me in 1893 and 1896 was, I believe, far beyond anything 

 obtained before. 



" The facts are as follows. With the 6-inch prismatic camera 

 used in 1893, the photographed spectrum was 3'i inches long 

 from D to K. With the 9-inch, which it was proposed to 

 employ in 1896, the corresponding length of spectrum was 3 '9 

 inches ; while with the 3-inch prismatic camera actually used 

 in Novaya Zemlya, the spectrum was 2-9 inches long from D 

 to K. 



* ' I believe the next highest dispersion obtained before was by 

 Captain Hills in 1893, ^^^ by Dr. Schuster in 1886. Data are 

 not available for exactly comparing the dimensions of the 

 spectra then photographed with those stated above, but they 

 were certainly considerably smaller in both cases. 



" The imperative necessity for this increased dispersion may 

 be gathered from the following facts concerning the spectrum 

 of iron which I have best studied, and on which I have 

 thousands of unpublished observations to compare with an 

 eclipse spectrum when we can get one on a sufficient scale. 



"Taking Rowland's Hnes, it maybe generally stated that, 

 on an average, one occurs at every 6/10 of a tenth-metre, the 

 unit of measurement generally employed in such matters. 

 With the dispersion in my photographs— the greatest so far ob- 

 tained, as I have explained — we do not feel ourselves justified 

 in assuming a greater accuracy than 5/10 of a unit. Evidently 

 then, so far as this line of work alone is concerned, we can 

 make no definite statements as to the presence or absence of 

 iron lines in the eclipse photographs. 



" So far as I am aware, no observations with the slit spectro- 

 scope will enable us to determine with any kind of exactness 

 the relative composition of the successive layers of the sun's 

 gaseous envelope. The difficulty chiefly arises, as I pointed 

 out in 1882, from the fact that we have to deal with the pro- 

 jection of a sphere surrounded by vapours, and not with a 

 section. 



"On the other hand, the photographs taken with the pris- 

 matic cameras in 1893, ^n^ during the last eclipse, show clearly 

 that there are essential differences in the composition of the 

 envelopes at different levels, and the limits of various layers 

 are indicated ; but the dispersion is too small to enable us to 

 define the chemical origins of the layers with sufficient cer- 

 tainty. A full statement of the evidence upon this point is 

 included in the report on the results obtained with the prismatic 

 cameras in 1893, which is now in the press.^ 



" (2) The prismatic camera has enabled us to photograph radia- 

 tionsat many different wave-lengths in the spectrum of the corona, 

 differentiating them absolutely from the radiations of the chromo- 

 sphere and prominences. This is a gigantic advance. But, in 

 the prismatic camera photographs, the indications, except in the 

 case of the 1474 ring, and two or three others, are very dim. 



"It is important, therefore, to employ an integrating spectro- 

 scope of large dimensions to attempt to get stronger indications 

 of these radiations by utilising the greater area of the corona, 

 which of course the prismatic camera cannot do. 

 1 Continued from page 178. 

 • This has since been published, and I shall refer to it later. 



"(3) I may say, roughly, that in the (still unpubli.shed) spec- 

 trum of the chromosphere obtained in 1893 and 1896, we deal 

 with less than 10 per cent, of the Fraunhofer lines. It is of the 

 first importance, then, to search for the others. I certainly saw 

 some of them in 1882, but a very special inquiry is necessary. 

 This I therefore include in my programme. These lines are 

 certain to be dim, otherwise we should have photographed them 

 already. The tendency of the observations of 1893 and 1896 is 

 to show that they will be found in all probability above the 

 chromospheric layer we have photographed, and associated with 

 the coronal layers, of which we have photographed a few of the 

 brightest radiations. 



"The thicknesses of the chromospheric layers have been : — 



H and K ... 



G 



4471 



Strontium line 4077 



Iron triplet 



Shortest arcs of Fe, &c. 



1893. 1896. 



less than 5000 miles. 5000 miles. 



3000 ,, 



,, 3000 ,, 



500 ,, 



500 ,, 



500 



90 miles. 



"Beyond the dark moon, both in 1893 and 1896, we have 

 indications of luminosity in the prismatic camera photographs, 

 but no final statement can be made as to its origin. 



" This gives us the spectrum of a part of the solar atmosphere 

 at a great height : — 



1893 ... 22,500 miles to 600,000 miles. 



1896 ... 14,000 ,, to a height not yet determined. 



" This, therefore, indicates a region, some 10,000 of miles in 

 thickness, to be also explored, and the blank in the photo- 

 graphic evidence so far obtained suggests that eye observa- 

 tions must be employed. 



" It will be seen from the above statement that the three parts- 

 of the proposed inquiry are all strictly connected, and that to 

 employ any one of them without the others would greatly weaken 

 the attack." 



It will have been gathered that the chief object of the 

 above observations is to determine the chemical and 

 physical conditions of that part of the sun's atmosphere 

 just above the photosphere, and therefore including the 

 chromosphere. 



Why so much importance is attached to such observ- 

 ations during eclipses, is that ordinary daily observations 

 on the uneclipsed sun, although they carry us far, do not 

 carry us far enough. 



The results once obtained are not limited to the sun, 

 they find their application in the study of every star in 

 the heavens ; it is, indeed, now recognised that observ- 

 ations of eclipses, such as those made in 1882, 1893, and 

 1896, provide us with a series of facts with which to 

 approach the question of the absorption phenomena 

 presented by the stars, and the whole question of the 

 classification of stars depends almost absolutely upon 

 their absorption phenomena. 



In many of these bodies the atmosphere may be 

 miUions of miles high ; in each star the chemical sub- 

 stances in the hottest and coolest portions may be vastly 

 different ; the region, therefore, in which the absorption 

 takes place which, spectroscopically, enables us to dis- 

 criminate star from star, must be accurately known 

 before we can obtain the greatest amount of information 

 from our inquiries. 



I may say that for some time I was of opinion that in 

 the sun many of the darkest lines indicated absorptions 

 high up in the atmosphere, for the reason that the bright 

 continuous spectrum of the lower levels might have an 

 important effect upon line absorption phenomena by 

 superposing radiation, and so diminishing the initial 

 absorption. The observations of the eclipse of 1893, 

 however, indicate that this opinion is probably only 

 strictly true when the strata of the sun's atmosphere 

 not too high above the photosphere are considered. 



If we are justified in arguing from a star with a photo- 

 sphere as well developed as that of the sun to one in 

 which it is in all probability much less marked in con- 



NO. 1449. VOL. 56] 



