1882.] Photographing the Sola? 1 Corona without an Eclipse. 411 



be made to permit of the detection of the more ordinary changes 

 which are doubtlessly taking place in it. 



These considerations induced me not to attempt eye-observations, 

 bnt from the first to use photography, which possesses extreme sensi- 

 tiveness in the discrimination of minute differences of illumination, 

 and also the enormous advantage of furnishing a permanent record 

 from an instantaneous exposure of the most complex forms. I have 

 satisfied myself by some laboratory experiments that under suitable 

 conditions of exposure and development a photographic plate can be 

 made to record minute differences of illumination existing in different 

 parts of a bright object, such as a sheet of drawing paper, which are 

 so subtile as to be at the very limit of the power of recognition of a 

 trained eye, and even, as it appeared to me, those which surpass thafc 

 limit. 



My first attempts at photographing the corona were made with 

 photographic lenses, but uncertainty as to the state of correction of 

 their chromatic aberration for this part of the spectrum, as well as 

 some other probable sources of error which I wished to avoid, led me 

 to make use of a reflecting telescope of the Newtonian form. The 

 telescope is by Short, with speculum of 6 inches diameter, and about 

 3^ feet focal length. A small photographic camera was fastened on 

 the side of the telescope tube, and the image of the sun after reflection 

 by the small plane speculum was brought to focus on tbe ground glass. 

 The absorptive media were placed immediately in front of the sensitive 

 film, as in that position they would produce the least optical distur- 

 bance. Before the end of the telescope was fixed a shutter of adjust- 

 able rapidity which reduced the aperture to 3 inches. This was 

 connected with the telescope tube by a short tube of black velvet for 

 the purpose of preventing vibrations from the moving shutter reaching 

 the telescope. On account of the shortness of the exposures it was 

 not necessary to give motion to the telescope. 



It was now necessary to find an absorptive medium which would 

 limit the light received by the plate to the portion of the spectrum 

 from about G to H. There is a violet (pot) glass made, which prac- 

 tically does this. I had a number of pieces of this glass ground and 

 polished on the surfaces. Three or four of these could be used 

 together, castor-oil being placed between the pieces to diminish the 

 reflection of light at their surfaces. Some inconvenience was found 

 from small imperfections within the glass, and it would be desirable 

 in any future experiments to have a larger supply of this glass, from 

 which more perfect pieces might be selected. 



In my later experiments I used a strong and newly made solution 

 'of potassic permanganate, in a glass cell with carefully polished sides. 

 This may be considered as restricting the light to the desired range of 

 wave-length, since light transmitted by this substance in the less 



