June 1, 1898.] 



KNOWLEDGE. 



131 



This beautiful phenomenon can therefore only be seen or 

 photographed at the momenta of disappearance or reap- 

 pearance of the photosphere at the beginning and end of the 

 total phase respectively. Under the average conditions of 

 an eclipse, perhaps not more than two seconds are available 

 just as totality comes on in which the flash spectrum may 

 be photographed in its full splendour ; and if the observer 

 is discerning enough to know exactly when the sun is 

 going to burst out again at the end of totality, he wiU 

 have another two seconds in which to expose a second 

 plate ! 



It has been estimated that our opportunities for studying 

 the corona do not amount in the aggregate to more than 

 some hours per century, and the progress of knowledge is, 

 in consequence, not rapid. How long it will take to 

 unravel the mysteries of the flash spectrum it is bard to 

 say, seeing that the time available for its study must be 

 reckoned in minutes per century. This method of estima- 

 tion is, however, certainly unjust to the photographic 

 plate, which enables us to study at leisure so very transient 

 a phenomenon. 



A great variety of photogi'aphic spectroscopes were used 

 at the recent eclipse, but all may be classed under two 

 heads, viz., slit spectrographs and prismatic cameras. 

 The great advantages possessed by the latter for eclipse 

 work were first pointed out by Sir Norman Lockyer, who 

 employed them with great success at the eclipse of 1893, 

 and again in 1896, when Mr. Shackleton first succeeded 

 in photographing the flash spectrum at his station in 

 Novaya Zemlia. The crowning success for the prismatic 

 camera is the splendid photogi-aph of the flash which Sir 

 Norman Lockyer has obtained at the recent eclipse with 

 his six-inch instrument in the hands of Mr. Fowler. 



The photographs which accompany this article were 

 obtained at the recent eclipse with a small instrument of 

 this class, which I constructed in 1896 for the eclipse of 

 that year. Without going into details as to the design of 

 this particular instrument, I may say generally that the 

 prismatic camera is the simplest of all spectroscopic appli- 

 ances. It consists essentially of a prism placed in front 

 of a camera lens. There is no slit or collimator, which in 

 the ordinary spectroscope are used to give purity to the 

 spectrum, and consequently it is not possible to photograph 

 spectra from extended sources of light, such as the disc of 

 the sun. But the prismatic camera is particularly well 

 adapted for photographing the spectrum of the solar 

 atmosphere during an eclipse, because, as before explained, 

 when the disc is enth-ely covered by the moon there 

 remains a thin crescent of light due to the layer of gases 

 outside, and which acts the part of a curved slit. If the 

 moon and sun were precisely equal in apparent size this 

 would extend all round in a ring, and would produce con- 

 siderable confusion in the spectral images ; but under 

 ordinary circumstances the moon is slightly the larger in 

 angular diameter, so that early in the total phase the 

 chromospheric gases appear as a half circle or crescent on 

 one side only, while later on this is in turn eclipsed and 

 the opposite portion is uncovered. 



In the succession of photographs given in the plate it 

 will be noticed that the spectral images of these crescents 

 exhibit this change, which occurs about the time of mid- 

 totality (between Nos. i and 7). It will of course be 

 understood that, owing to the essential nature of gaseous 

 radiation, and to the wonderfully complete mixture of gases 

 existing at the base of the chromosphere, an enormous 

 number of distinct images or spectrum '• lines " are shown 

 in the photographs taken near second and third contact. 

 The pair of very strong images to the left hand of the 

 central portion of each spectrum are those due to the well- 



known radiations of calcium vapour, namely, H and K ; 

 they give complete images of the chromosphere and pro- 

 minences — or, rather, as much as was uncovered by the 

 moon at the time each photograph was taken. 



Before proceeding to describe in detail the results 

 obtained, I will give a brief description of the arrange- 

 ments I made for this work at the camp of the British 

 Astronomical Association stationed at Talni. 



My plan of work was first to obtain a series of ten 

 photographs with the prismatic camera during totality, 

 and including, if possible, the flash spectrum at both 

 second and third contacts ; secondly, to photograph the 

 flash spectrum on a larger scale with a large slitless 

 spectrograph attached to a six-inch telescope; and, thirdly, 

 to expose a single plate to the spectrum of the corona 

 during the whole time of totality by means of a slit spectro- 

 graph containing quartz prisms. 



Besides these three photographic instruments I had 

 available a four- inch polar heliostat, kindly placed at my 

 disposal by Mr. W. H. Maw ; and a three and a quarter 

 inch equatorial telescope provided with a powerful solar 

 spectroscope, with which I intended to actually observe 

 the flash of bright lines at second contact, and thus 

 determine the exact moment when to expose the photo- 

 graphic instruments. 



The heliostat, which I arranged with two four-inch 

 mirrors instead of one, was used to supply light to the 

 prismatic camera and to the slit spectrograph, both these 

 being mounted on fixed supports firmly bedded in cement. 

 The six-inch telescope with its spectrograph was mounted 

 equatorially, but without any driving gear, and was 

 pointed directly at the eclipsed sun. 



As there would be no one available on the day of the 

 eclipse to assist me, the three photographic instruments 

 bad to be arranged with their exposing shutters near 

 together, so that I could work them all while seated near 

 the telespectroscope. 



The diagram will show better than any description 

 the disposition of the various instruments within the 

 observing hut. 



H- SOUTH. 



Px.Ay OF Obsebving Hut. 



E. Equatorial Telespectroscope ; H. Heliostat ; PC. Prismatic 

 Camera ; SG. Spectrograph with Six-inch Object Glass ; ss. Slit 

 Spectrograph with two Quartz Prisms. 



Thanks to the facilities afforded by the Indian (.iovern- 

 ment in providing workmen and materials, and to the 



