September 1, 1890.] 



KNOWLEDGE 



211 



into it, so far as appears, but the very minuteness of 

 the punctures seems to be the cause of the irritation, 

 just as a prick with an exceedingly fine needle often causes 

 intense pain. 



{To be conthiuecl .) 



ON SOME RECENT ADVANCES IN THE MAP- 

 PING OF THE SOLAR SPECTRUM. 



By a. C. Rany.^rd. 



Tlli'j third volume of the Anmds of the Xice i Ibserro- 

 tiiry has just appeared, accompanied by an atlas 

 of 17 plates, in which M. Thollon's beautiful 

 drawings of the solar spectrum have been repre- 

 sented by carefully executed steel plates, engraved 

 and published at the expense of ^I. Bischoffsheim. The 

 atlas represents the fruits of between six and seven years 

 close application of a man of remarkable ingenuity and 

 exceptional gifts ; and the engraver, with the aid of 

 M. Perrotin, the Director of the Nice Observatory, has 

 been occu|iied for more than three years, since the death 

 of M. 'I'hdlldii, in reproducing the drawings. 



M. Thdiloii's map of the spectrum was made with a 

 large bisulphide of carbon spectroscope kept at an even 

 temperature by water circulating -within the table on which 

 it was moiuited, and also within a large double-walled 

 metal box which descended from the ceiling and covered 

 the instrument. The light of the sun was thrown from a 

 heliostat on to the slit of the eolhmator which passed 

 through one side of the metal box, and the observer looked 

 through a telescope passing through the other side of the 

 box. All adjustments of the prisms and other internal 

 parts were made by rods and cords passing through the 

 sides of the box, while cold water was constantly kept cir- 

 culating around the instrument. With this spectroscope 

 M. Thollon made his chart of the spectrum extending from 

 the Great A group at the red end of the \-isible spectrum 

 to /* in the blue green. The maps put together are more 

 than ten metres long, and contain more than three thou- 

 sand lines. Not only was the position of each line care- 

 fully determined, but its tliifkness and blackness and the 

 character of its nebulous edges were portrayed with con- 

 scientious accnrac}', and its behaviour was studied when 

 the sun was at different altitudes. Each portion of the 

 spectrum is represented by four bands, the upper one of 

 which represents tlie solar spectrum as observed when the 

 sun is at a distance of 80° from the zenith and the air is 

 neither very dry nor very damp. The second band repre- 

 sents the spectrum as seen when the sun is (iO° from the 

 zenith and the air is very damp. The third band repre- 

 sents the spectrum as seen when the sun is G0° from the 

 zenith and the air is very dry. The fourth band contains 

 all the lines of exclusively solar origin, and lias been con- 

 structed by prolonging tlie lines which remain unaltered 

 under varying atniosplieric conditions. 



At the time when ^I. Thollon commenced his mapping 

 of the spectrum, and uj) to the time of his dcatli, he did 

 not imagine that photographs would so soon be taken 

 which would show all tlie detail which can be seen by the 

 eye with a spectroscope of large dispersion. It would 

 have been impossible to photograph with his bisulphide of 

 carbon prisms, though they gave a much more brilliant 

 spectrum than can be seen with similar dispersion and a 

 grating spectroscope, for during the time necessary to 

 expose a plate slow changes of temperature would sufH- 

 ciently alter the refractive index of the bisulphide of 

 carbon to shift the place of the lines on the sensitive 



plate, and the photographic action of light in a great part 

 of the region mapped by M. Thollon was very slow with 

 the processes then known. 



In 1K80 Prof. H. A. Rowland, of Jolms Hopkms Univer- 

 sity, a brilliant young American physicist and mathema- 

 tician, devised a plan by which he was enabled to 

 overcome the chief practical difficulties which mecha- 

 nicians had up to that time encountered in cutting accurate 

 micrometer screws. This enabled him to succeed in 

 ruling diifi'action gratings containing up to 13,000 lines to 

 the mch, without an appreciable periodic error amounting 

 to the hundred-thousandth part of an inch, and it 

 almost entirely got rid of the ghosts which were so ft-e- 

 quently seen overlying the spectra thrown by the earlier 

 gratings. These were due to the recurrence of groups of 

 narrower and broader spaces at periodic intervals which 

 gave their own spectrum, and caused ghosts of the more 

 prominent lines sometimes on both sides of them. 



In 1881 the happy idea occurred to Prof. Rowland that 

 a difiVaction grating might be ruled on a spherical surface, 

 and that by this means the spectrum might be brought to a 

 focus without any lenses, thus reducing the spectroscope 

 to its simplest form, and avoiding the cumbersome 

 adjuncts of collimator and viewing telescope, as well as 

 greatly facilitating physical researches in the parts of the 

 spectrum where it had hitherto been found necessary to 

 use rock salt or quartz lenses. 



In 188G Prof. Rowland completed and published copies 

 of a photographic map of the normal solar spectrum, taken 

 with one of his concave gratings of six inches in diameter 

 and 21 i feet rachus. It extended fi-om wave-length 3680 

 to 5790"; that is, from above H donsm nearly as far as D. 

 The region in the neighbourhood of H is on four tunes the 

 scale of Angstrom's map ; the region at the less refrangible 

 end is on twice the scale, and the intermediate portion is on 

 three times the scale of Angstrom. The definition was so 

 good that 1474, h^, and b^, are shown clearly as double, 

 and E, though not distinctly split, can be easily recognised 

 as double. In 1889 Prof. Rowland published a new series 

 of photographs of the spectrum, extending from wave-length 

 3000 to wave-length 6950, which showed a distinct 

 advance in definition on the former series. 



Another very marked step in advance has been made liy 

 Mr. George Higgs, of Liverpool, to whom we are indebted for 

 the beautiful photographs of the portions of the solar spec- 

 trum which accompany this piaper. He has made use of one 

 of Prof. Rowland's concave gratings of four inches diameter, 

 with a radius of curvature of 10 feet 2 inches, which gives 

 less than half the dispersion of the 21i-feet grating used 

 by Prof. Rowland ; but the photographs he has obtained are 

 decidedly sharper, and show more lines than those of Prof. 

 Rowland. For instance, Mr. Higgs counts 22 lines be- 

 tween I) J and D,_, on his original negatives. I have recently 

 made a visit to T^iverpool to endeavour to learn something 

 of the means by which Mr. Higgs has obtained such sharp 

 definition. It seems to be owing to extreme care in 

 making his adjustments, and to the very fine steel-jawed 

 slit which Mr. Higgs has made for his instrument. He has 

 been 0('cupied for some years with exiieriments on the pho- 

 tography of the solar spectrum, and has made an extended 

 series of experiments on the dyes for sensitive plates which 

 enable photographs to l^e taken in the infi-a red and ultra 

 violet regions, and he now proposes to publish a photogra- 

 phic map of the spectrum, extending from wave-length 

 2090 to wave-length 8500, Each section will be photo- 

 graphed coincidentally \\\t\\ some other portion of the 

 spectrum belonging to a different order, so as to give a 

 natural scale, by which any possessor of the photoari-aph 

 may determine the relative wave-lengths of the lines 



