June 1, 1896.] 



KNOWLEDGE. 



133 



scope by simply directing the collimator to the moon's 

 centre, or as an analyzing spectroscope by placing the slit 

 in the focal plane of a telescopic objective or condenser. 

 In the first case the spectra of corona, prominences, and 

 chromosphere will all be superposed, so that in the absence 

 of other evidence the spectra of the different parts could 

 not be separated. The use of a condensing lens enables 

 the observer to localize some particular region of the 

 corona upon the slit, and its spectrum may then be studied 

 apart from that of its neighbours. 



Spectroscopes without slit or collimator were first 

 employed for eclipse observation by Respighi and Lockyer, 

 in 1871. The principle of this method is very simple. 

 In an ordinary spectroscope the function of the coUimating 

 lens is to render parallel the rays proceeding from the slit ; 

 and the " lines" which one usually associates with spectra 

 are simply images of the slit. If the ordinary slit be 

 replaced by a circular one the spectrum lines become 

 spectrum rings ; indeed, the shape of the slit in all cases 

 is reproduced in the " lines." The rays proceeding from 

 the sun are already parallel, so that a collimating lens is 

 superfluous, and the eclipse itself is at once the source of 

 light and the slit. The eclipsed sun is in reality a number 

 of independent slits : one — a complete ring — corresponding 

 to the corona, and others of irregular form corresponding 

 to the different parts of the chromosphere and prominences. 

 Each "line" in the spectrum of a prominence is repre- 

 sented by a picture of the prominence, and if different 

 vapours are unequally distributed these images will differ. 



^Yhen the slitless spectroscope is employed for photo- 

 graphy it becomes a 

 " prismatic camera," 

 and takes the form of a 

 large prism placed in 

 front of the objective of a 

 photographic telescope. 

 Such an instrument is 

 shown in Fig. 2, and a 

 photograph taken with it 

 in Fig. 8, both of which 

 we owe to the kindness 

 of Prof. Lockyer. As a 

 single exposure gives Fio. 4.— Prominences in K 



the spectrum of every 

 part of the solar sur- 

 roundings, the prismatic camera provides a very complete 

 record of the spectroscopic appearances during an eclipse. 

 For a considerable interval before and after totality the 

 sky illumination is sufiiciently reduced to show the 

 chromosphere spectrum near the cusps of the crescent 

 sun, and very valuable photographs may therefore be 

 taken out of totality. Incidentally it turns out that the 

 prismatic caruera is the best possible instrument for 

 photographing the solar prominences. This is illustrated 

 in Fig. 4, which is an enlarged view of a group of 

 prominences photographed in K light during the eclipse of 

 1893. 



Captain Hills has provided himself with a very fine slit 

 spectroscope, having four quartz prisms and a condensing 

 lens of four and three-quarter inches aperture, with which 

 he hopes to photograph the spectrum of the corona. The 

 instrument will be fixed on a horizontal base, and the 

 rays of Ught from the eclipsed sun will be reflected into 

 it by a heliostat of the ordinary form. 



In Norway the principal spectroscope to be employed 

 by Prof. Lockyer and the writer is a prismatic camera of 

 six inches aperture, which was very successfully used in 

 Africa in 1898. The instrument is mounted as an equa- 

 torial, and the arrangements provide for the exposure of a 



large number of plates in quick succession. An integrating 

 spectroscope will also be employed by Prof. Lockyer. 



We understand that Dr. Copeland wiU be armed with 

 a prismatic camera, whOe Mr. Evershed will employ 

 spectroscopes of both forms. The Russian party at 

 Olekminsk will also be provided with a slit spectroscope 

 arranged to photograph the coronal spectrum, and this 

 method will probably be followed by M. Deslandrea in 

 ■lapan. 



As the photographic plates cannot yet be relied upon tu 

 completely delineate the less refrangible parts of the 

 spectrum under the special circumstances of an eclipse, 

 they will be supplemented by visual observations. Mr. 

 Maunder intends to use a slitless spectroscope to study 

 specially the distribution and extension of the green ring 

 corresponding to 1471 K, and it is hoped that other 

 observers will similarly record the appearances of the 

 yellow line of helium and the red line of hydrogen. 



Apart from adding to our scanty knowledge of the un- 

 doubted coronal spectrum, and its possible variation from 

 one eclipse to another, one of the chief points on which it 

 is hoped that the spectroscopic work will enlighten us is 

 the location in the sun's atmosphere of the vapours which 

 produce the Fraunhofer lines. There is httle doubt that 

 the base of the chromosphere — the so-called " reversing 

 layer " — is rich in bright lines, but the precise relation of 

 these to the dark ones of the Fraunhofer spectrum is by 

 no means understood. 



Among the more special investigations which will pro- 

 bably be undertaken one may mention that Dr. Downing 



r.isht. Prismatic Camera, 1S93. 



wUl make polariscopic observations, with the view of 

 ascertaining the proportion of reflected to intrinsic light 

 in the corona. 



Photometric observations, having for their object the 

 determination of the general brightness of the corona, will 

 probably also be made at some of the observing stations, 

 easy methods for doing this having been suggested by Mr. 

 Maunder and Mr. Lunt. Mr. Crommelin has drawn 

 attention to the desirability of recording the attendant 

 phenomena, such as the mysterious " shadow bands," and 

 some of the observers who have no telescopes will doubt- 

 less keep a look-out for such appearances. 



The Russian observers at Enontekis will study the 

 relation of the coronal spectrum to that of helium, and 

 there is little doubt that M. Deslandres will endeavour to 

 test the truth of his conclusion from the observations of 

 1898, that the corona shares in the general solar rotation. 

 Prof. Voznesensky's object during the eclipse will be to 

 investigate the meteorological effects which accompany 

 the temporary obscuration of the sun's hght and heat. 



From what has been said it is evident that there will be 

 no lack of observers during the forthcoming eclipse, and 

 all friends of science will wish them the utmost success in 

 the various researches undertaken. 



