314 
CAPTAIN L. DARWIN, DR. A. SCHUSTER, AND MR. E. W. MAUNDER 
the air glare. In fig. 2, let the lines round the Sun represent lines of equal intensity 
of sky light, the corona being supposed to be non-existent. In fig. 3, let the Sun be 
eclipsed, and the lines represent lines of equal intensity of corona light. Place one 
image on the other, and the lines of equal intensity of the combined light of corona 
and air glare will be as shown in tig. 4, in which it will be seen how materially the 
form of the corona appears to be altered. It is to be noted also that an alteration in 
the state of the atmosphere by altering the ratio of air glare to corona light would 
appear to alter the form of the corona as shown on the Sun light photographs. 
For the purposes of the following discussion of the effect of combining the sky and 
corona lights on one picture, it is assumed that this 2 :)articular difficulty does not 
exist; that is to say, that the air glare is uniform, or, what comes to nearly the same 
thing, that we are discussing a radial inequality of the corona. In dealing numerically 
with this problem, very little trust must be placed in figures used; but, however 
faulty they may be, they may, at all events, help to make the consideration of the 
subject more easy. 
Captain Abney informs me his experiments with regard to sky light when compared 
with the results obtained by Professor Thorpe at this eclipse show that .at 30' from 
the Sun’s limb the corona, as seen at Grenada, was about 60 times less brilliant than 
the air glare at the same distance from the Sun’s limb, as seen at .South Kensington 
under the most favourable circumstances, that is, with the darkest sky measured. As 
the sky on this occasion at South Kensington was probably darker than the sky at 
Grenada, the ratio at this latter place was probably greater than 1 to 60 ; but, on 
the other hand, with a clearer sky the corona would have been brighter and the ratio 
therefore less. Hence the ratio of 1 to 60 is about half Avay betAveen the ratio under 
fiivourable circumstances at South Kensington and the ratio at Grenada near the 
time of the eclipse. For the purposes of argument, let it be assumed that at Grenada 
the ratio of the light of the corona to the light of the sky at 30' from the Sun’s limb 
was as 1 to 50, a ratio, if the above figures are correct, no doubt considerably less than 
the true ratio. 
Captain Abney Also informs me that experiments he has made show that an abrupt 
change of ^ per cent, in the density of a photograph is about the minimum change of 
shade that can be seen by the eye ; or, in other Avoids, that a photograph may be 
regarded as a draAving in Avhich only 200 different shades can be used. 
It has already been shoAvn that to obtain the best results a photograph at a half or 
