75 8 THE POPULAR SCIENCE MONTHLY. 



this light falls within the range of vision, the eye is less sensitive to 

 small differences of illumination near this limit of its power. This 

 consideration and some others led me to look to photography for aid, 

 for it is possible by certain technical methods to accentuate the ex- 

 treme sensitiveness of a photographic plate for minute differences of 

 illumination. [A cardboard, on which a corona had been painted by 

 so thin a wash of Chinese white that it was invisible to the audience, 

 had been photographed. The photograph thrown upon the screen 

 showed the corona plainly.] This cardboard represents the state of 

 things in the sky about the sun. The painted corona is brighter than 

 the cardboard, but our eyes are too dull to see it. In like manner the 

 part of the sky near the sun where there is a background of corona is 

 brighter than the adjoining parts where there is no corona behind, 

 but not in a degree sufficiently great for our eyes to detect the dif- 

 ference. 



A photographic plate possesses another and enormous advantage 

 over the eye, in that it is able to furnish a permanent record of the most 

 cojnplex forms from an instantaneous exposure. 



In my earlier experiments the necessary isolation of violet light 

 was obtained by interposing a screen of colored glass or a cell contain- 

 ing potassic permanganate. The possible coming of false light upon 

 the sensitive plate from the glass sides of the cell, as well as from 

 precipitation due to the decomposition of the potassic permanganate 

 under the sun's light, led me to seek to obtain the necessary light-selec- 

 tion in the film itself. Captain Abney had shown that argentic bro- 

 mide, iodide, and chloride, differ greatly in the kind of light to which 

 they are most sensitive. The chloride is most strongly affected by 

 violet light from h to a little beyond K. It was found possible by 

 making use of this selective action of argentic chloride to do away 

 with an absorptive medium. To prevent reflected light, the back of 

 the plate was covered with asphaltum varnish, and frequently a small 

 metal disk a little larger than the sun's image was interposed in front 

 of the plate to cut off the sun's direct light. 



The next consideration was as to the optical means by which an 

 image of the sun, as free as possible from imperfections of any kind, 

 could be formed upon the plate. For several obvious reasons the use 

 of lenses was given up, and I turned to reflection from a mirror of 

 speculum metal. My first experiments w r ere made with a Newtonian 

 telescope by Short. With this instrument, during the summer of 1882, 

 about twenty plates were taken on different days, in all of which coro- 

 nal forms are 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 Captain Abney, there seemed to be 

 good ground to hope that the corona had really been obtained on the 

 plates. [One of these negatives, obtained in August, 1882, was shown 

 upon the screen.] 



