204 Mr. William Huggins [Feb. 20, 



if our eyes were more sensitive to small differences of illumination of 

 adjacent areas. My friend Mr. John Brett, A.E.A. tells me that he 

 is able to see the corona in a telescope of low power. 



The spectroscopic method by which the prominences can be seen 

 fails because a part only of the coronal light is resolved by the prism 

 into bright lines, and of these lines no one is sufficiently bright, and 

 co-extensive with the corona, to enable us to see the corona by its 

 light, as the prominences may be seen by the red, the blue, or the 

 green line of hydrogen. 



The corona sends to us light of three kinds. (1) Light which the 

 prism resolves into bright lines, which has been emitted by luminous 

 gas. (2) Light which gives a continuous spectrum, which has come 

 from incandescent liquid or solid matter. (3) Reflected sunlight, 

 which M. Janssen considers to form the fundamental part of the 

 coronal light. 



The problem to be solved was how to disentangle the coronal 

 light from the air-glare mixed up with it, or in other words how to 

 give such an advantage to the coronal light that it might hold its 

 own sufficiently for our eyes to distinguish the corona from the bright 

 sky. 



When the report reached this country in the summer of 1882 

 that photographs of the spectrum of the corona taken during the eclipse 

 in Egypt showed that the coronal light seen from the earth, as a whole 

 is strong in the violet region, it seemed to me probable that if by some 

 method of selective absorption this kind of light were isolated, then 

 when viewed by this kind of light alone the corona might be at a suffi- 

 cient advantage relatively to the air-glare to become visible. Though 

 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 

 extreme 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 difference. 



A photographic plate i^ossesses another and enormous advantage 

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

 most complex forms from an instantaneous exposure. 



In my earlier experiments the necessary isolation of violet light 

 was obtained by interposing a screen of coloured glass or a cell 

 containing potassic permanganate. The possible coming of false 

 light upon the sensitive plute from the glass sides of the cell, as well 



