1885.] on the Solar Corona. 205 



as from precipitation due to the decomposition of the potassic per- 

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

 light-selection in the film itself. Captain Abney had shown that 

 argentic bromide, 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 fre- 

 quently a small metal disc 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 were 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 

 coronal 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.] 



In the spring of the following year, 1883, the attack upon the 

 corona was carried on with a more suitable apparatus. The Misses 

 Lassell were kind enough to lend me a seven-foot Newtonian 

 telescope made by Mr. Lassell which possesses great perfection of 

 figure and retains still its fine polish. For the purpose of avoiding 

 the disadvantage of a second reflection from the small mirror, and 

 also of reducing the aperture to Sh inches, which gives a more 

 manageable amount of light, I adopted the arrangement of the 

 instrument which is shown in the following woodcut. 



SHUTTER 



7? 



The speculum b remains in its place at the end of the tube a, a, 

 by which the mechanical inconvenience of tilting the speculum 

 within in the tube as in the ordinary form of the Herschelian tele- 

 scope is avoided. 



The small plane speculum and the arm carrying it were removed. 

 The open end of the tube is fitted with a mahogany cover. In this 



