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IX. Wcwe-hmjth Determinations and General Results obtained from a Detailed 
Examination of Spectra Photoe/raplied at tie Solar Eclipse of January 22, 
1898. 
By John Evershed, F.R.A.S. 
Comriiimicated hy Dr. A. A. Rambaut, F.R.S. 
Keceived December 12, 1900,—Read January 17, 1901 
[Plates 10-12.] 
The results are here given of a detailed study and measurement of a series ot ten 
spectra, photographed by me with a small prismatic camera, at the eclipse camp of the 
British Astronomical Association stationed at Talni, India. 
The instrument referred to had an aperture of 50 millims., and a focal length of 
about 890 millims. It was fitted with two crown-glass prisms, each of 60° angle, 
l^laced in front of an ordinary visual objective, the component lenses of which were 
slightly separated in order to shorten tlie focus of the ultra-violet rays relatively to 
those of the visible spectrum. 
[The prisms, which were set at minimum deviation for K, were originally intended 
for use in a spectroheliograph designed by Professor PIale, and were used by him in 
an attempt to photograph the corona in sunlight. They were made by the Zeiss 
Optical Company, of carefully selected glass, and are 59 millims. in height and 65 
millims. measured on the faces, giving an effective aperture of about 42 millims. The 
total thickness of glass, or mean length of path traversed by the rays in glass, is 
therefore 65 millims. —March 9, 1901.] 
The camera box was fitted with a long sliding plate-holder, moved by rack and 
pinion across the field of the lens and at right angles to the length of the spectrum. 
By this means it was possible to change the plates and make the successive exposures 
at very short intervals of time.^ 
In all the eclipse negatives obtained with this instrument, the extension of the 
spectra in the ultra-violet is greater than had been anticipated, and the density of 
the silver de})Osit is surprisingly uniform throughout the spectrum. In this connec¬ 
tion it is of interest to compare the eclijjse j)lates with a series of trial plates made 
* For a full description of the methods of working see ‘Report on the Indian Eclipse of 1898,’ 
published be the British Astronomical Association. 
(2!)5) 
le.ii.isoi. 
