812 Mr. W. H. M. Christie [April 22, 



glass of comparatively short focal length, and thus the total length 

 of the telescope is kept within manageable dimensions — 11 feet in 

 my case, instead of 40 feet as in the ordinary form. This combina- 

 tion is in fact an application of the well-known Barlow lens, and 

 forms what has since become known to photographers as the tele- 

 photo form. My instrument was the photographic telescope, of 

 9 inches aperture and 8 feet 6 inches focal length, presented some 

 years ago to the Greenwich Observatory by Sir Henry Thompson, 

 and to this was applied a tele-photo concave magnifier of 3 inches 

 diameter giving a solar image 4 inches in diameter, with a field of 

 view of 10 inches diameter (2 J diameters of the sun). 



The same so-called tele-photo form was also used for two smaller 

 telescopes of 4 inches aperture which gave a solar image 1 J inches in 

 diameter, each of these being combined with another photographic 

 telescope of 4 inches aperture and 62 inches focus (known as the 

 Abney lens) in a double tube. Thus each " double tube " gave two 

 photographs of the corona, large and small scale, the former to show 

 detail and the latter to give as great extension as possible. These 

 " double tubes" wore first used in the eclipse of 1893. In the recent 

 eclipse they were efiectively employed by Professor Turner at Sahdol, 

 and by Captain Lenox Conyngham, K,E., under Captain Hills' direc- 

 tion, at Pulgaon. 



Another important feature in the instrumental equij^ment was the 

 coelostat, a form of mounting a mirror devised by M. G. Lippmann 

 in 1895, and successfully used in the recent eclipse at three stations 

 (Sahdol, Pulgaon and Viziadrug) ; the observers being indebted to 

 Dr. Common for designing the instruments, supervising their con- 

 struction and, most important of all, supplying the large plane mirrors 

 (16 inches in diameter). 



Another new departure of much interest was Professor Burck- 

 halter's device for giving to each part of the corona the exact 

 exposure best suited to its brightness. The brightness of the inner 

 parts near the sun's limb is so overpowering, as contrasted with the 

 faintness of the outer streamers, that widely ditFerent exposures are 

 required to bring out their respective details, and thus it is necessary 

 to take a series of photographs, the combination of which should 

 represent the whole phenomenon. 



Professor Burckhalter arranges to get the whole on one plate by 

 giving exposures rapidly increasing from the sun's limb to the edge 

 of the field, this being effected by means of a slit of peculiar form in 

 a metal screen which rotates rapidly in front of the photographic 

 plate, and thus gives intermittent exjjosures of duration depending on 

 the width of the slit, which increases rapidly from the sun's limb 

 outwards. 



Another interesting instrument was that used by Mr. Thwaites at 

 Talni with a triple object-glass, 4i inches in diameter, of Cooke's 

 new form adapted both for visual observation and for photography. 



Valuable series of photographs of the corona were obtained with 



