On the Corona of the Sun. 



113 



In my early experiments I made use of coloured glass, or a cell 

 containing a coloured liqnid, for the purpose of isolating the violet 

 part of the spectrum,* hut afterwards I obtained the desired light- 

 selection in the film itself by the use of argentic chloride, which 

 Captain Abney had shown to be most strongly sensitive to light from 

 Ho a little beyond H.f Plates prepared with argentic chloride 

 possess a further advantage for this work in consequence of the 

 greater steepness of their gradations of density corresponding to 

 differences of light-action as compared with argentic bromide plates. 



"When very small differences of illumination only, existing close 

 about a body so enormously bright as the sun, have to be photographed, 

 the most minute precautions have to be taken to avoid false effects 

 upon the plate, which may arise from several causes. Lenses should 

 not be used to form the sun's image on the sensitive surface, in con- 

 sequence of possible false light about the image which may come from 

 outstanding aberrations, though they have been corrected for photo- 

 graphic work, and from reflections at the surfaces of the lenses. I 

 therefore employed a mirror of speculum metal. . Other necessary 

 precautions are described in my paper, namely, the position of the 

 shutter very near the focal plane ; protecting the sensitive surface 

 from the sun's direct light by a metal disk a little larger than the 

 sun's image ; placing before the apparatus a long tube fitted with 

 diaphragms to prevent light from the sky, excepting near the sun, 

 from entering the apparatus ; backing the plates with asphaltum 

 varnish ; and some others. J 



In my experiments in 1882 I used a Newtonian telescope by Short, 

 but afterwards a fine mirror made by the late Mr. Lassell, which was 

 so arranged that the image was formed directly upon the plate without 

 reflection from a second mirror. § 



About twenty plates were taken in 1882, in all of which an appear- 

 ance more or less coronal in character is to be seen about the sun's 

 image. After a very critical examination of these plates, in which I 

 was greatly helped by the kind assistance of Professor Stokes and of 

 Captain Abney, there seemed to be good ground to hope that the 



* "Proc. Roy. Soc," vol. xxxiv, pp. 411, 412. 

 f " Proc. Roy. Soc." vol. xxxiii, p. 175. 



t " Proc. Roy. Soc," vol. xxxiv, p. 409, also Report Brit. Ass., 1883, p. 348. See 

 also the photographic experiments of Dr. Lohse, "Yierteljarsschrift Ast. G-esell.," 

 Bd. xv, S. 134. Dr. Lohse sums up an account of his methods and results thus : — 

 " Es gelang aber dieselben (die Schwierigkeiten) zu uberwinden und Resultate zu 

 erhalten welche zu einer Fortsetzung der — hier freilich selten moglichen und mit 

 grosserem Vortheil in moglichst hober Lage anzustellenden — Experimenten ermu- 

 thigen." 



§ " I am indebted to Miss Lassell for the loan of a 7-foot Newtonian telescope made 

 by the late Mr. Lassell. The speculum, which is 7 inches in diameter, possesses 

 great perfection of figure, and still retains its original fine polish. I decided not to 



VOL. XXXIX. I 



