ON THE TOTAL SOLAR ECLIPSE OF AUGCST 29, 1886. 
315 
a third full density should be obtained. Let us assume the most favourable circum¬ 
stances, and that the development has brought the part of the picture under 
consideration up to half full density. Then, if there are only 200 possible shades 
in any photograph, there are, tlierefore, only 100 possible shades below this shade, 
which for convenience may be represented by 100. Let the light which caused this 
shade be also represented by 100. Now, if the shade on the photograph varied 
proportionately with the light, each of the 100 parts of light would be represented by 
one shade on the photograph, and a change of light less than 1 per cent, would not 
produce a change of shade on the photograph which would be visible to the eye. But 
there is no doubt that under certain conditions the shade on these photographs varies 
proportionately more rapidly than the light which it represents, and, judging from 
Captain Abney’s curves, before mentioned, it is not quite a fanciful supposition to 
assume that under the most favourable circumstances it varies twice as rapidly in 
parts of the photograph ; that is to say, that a change of ^ per cent, in the luminosity 
of the object is the minimum change which would be visible on tlie photograph. 
Now, in a photograph taken in Sun light of the sky at 30' from the Sun’s limb, the 
100 parts of light will consist of 98 of sky light and 2 of corona light. The sky light 
being assumed to be constant, the minimum visible change of -I- per cent, in the total 
light must be due to a change of 25 per cent, in the total light of the corona ; whereas, 
with a photograph taken during a total eclipse, on similar suppositions, a change of 
^ per cent, in the light of the corona will produce a visible change of shade on the 
photographs, or, in other words, the totality photographs will show 50 times as much 
detail as the Sun light photograj)hs. It will be observed that, for the purposes of 
comparison of the photographs taken in Sun light and during totality, neither an error 
in the assumption of the shade to which the photographs were developed, nor in the 
assumption of the ratio of change of light to change of shade, nor in the number of 
possible shades in a photograph, would vitiate the result, as they would apply equaily 
to both cases. 
It therefore appears to me most probable that, under all circumstances, by what¬ 
ever ratio the air glare is brighter than the corona, in very nearly the same ratio will 
the detail ol the corona be obliterated in a photograph taken during Sun light, as 
compared with one taken during a total eclipse ; that is to say , that, unless a change 
of shade in the corona were considerably more than 50 times as abrupt as the least 
change visible in the totality photographs at Grenada, it would be invisible in the 
photographs taken there in Sun light. 
This naturally leads to the question—Are there abrupt changes of shade in the 
corona, or does the light diminish gradually as the distance from the Sun increases ? 
As far as I have seen, an increase of exposure of the photographs taken during 
totality regularly brings with it an increased extension of the corona, as photographed ; 
that is to say, that the light of the corona gradually diminishes in intensity from the 
Sun outwards. The detail of the inner parts of the corona soon gets obliterated as 
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