308 ANNUAL OF SCIENTIFIC DISCOVERY. 



practically indispensable. In examining the spectrum of a group 

 of sun-spots, for instance, it is very much easier to move the 

 slit to the particular point we wish to observe than to move the 

 solar image by the tangent screws of the equatorial. The pro- 

 tuberances are so well seen through the F and 2796 (near G) 

 lines, that it is even possible to photograph them, though, 

 perhaps, not satisfactorily with so small a telescope as the one 

 at my command. Some experiments I have recently made 

 show that the time of exposure, with ordinary portrait collodion, 

 must be nearly 4 minutes, in order to produce images of a size 

 which would correspond to a picture of the solar disc about 2 

 inches in diameter. This length of exposure demands a more 

 perfect clock-work than my instrument possesses, and a more 

 accurate adjustment of the polar axis than it had during the 

 experiments, as well as a steadier condition of the atmosphere. 



" Thus far, therefore, I have not been able to produce anything 

 which could properly be called a good picture. Negatives have 

 been made which show clearly the presence and general form of 

 protuberances, but the definition of details is unsatisfactory. 

 This amount of success was reached upon September 28, when 

 impressions were obtained of 2 protuberances on the S.E. limb of 

 the sun, and, slight as this success was in itself, I consider it of 

 importance in showing the perfect feasibility of going much fur- 

 ther with more sensitive chemicals, more delicate adjustments, 

 and greater telescope power. I was aided in the experiments by 

 Mr. H. O. Ely, our local photographer, to whom are due my warm- 

 est acknowledgments for the interest, patience, ingenuity, and 

 skill, with which he assisted me. 



" We worked through the Hydrogen / line (2796 of KirchhofFs 

 scale), which, though very faint to the eye, was found to be de- 

 cidedly superior to F in actinic power. The photographic appa- 

 ratus employed consisted merely of a wooden tube, about 6 inches 

 long, attached at one end to the eye-piece of the spectroscope, and 

 at the other carrying a light frame. In this frame was placed a 

 small plate-holder, containing for a sensitive-plate an ordinary 

 microscope slide, 3 inches by 1. The image of the prominence, 

 seen through the open slit, is magnified and thrown upon this 

 plate by the eye-piece." 



PHOTOGRAPHING THE SUN. 



At a meeting of the American Academy, May 24, 1870, Profes- 

 sor Joseph Winlock exhibited a photograph of the sun taken with 

 a lens of 40 feet focus, and 4 inches aperture. As it is difficult to 

 place a tube of this length in an inclined position, it is laid hori- 

 zontally, and an image of the sun is reflected into it by a plane 

 mirror of unsilvered glass. When this mirror was blackened on 

 one side it became heated to such an extent as to shorten the 

 focus of the lens nearly 3 feet. The image obtained is about 4 

 inches in diameter, and is free from distortion produced by an 

 eye-piece. The exposure is instantaneous, and is effected by 



