348 ; REPORT—1883. 
potassic permanganate in solution, which were placed close to the sensitive 
surface, and between it and the shutter. I was much troubled by the 
rapid decomposition of the potassic permanganate under the influence of 
the sun’s light. When apparently clear to the eye, a lens revealed 
minute particles which precipitated themselves upon the glass plates of 
the cell, and gave an appearance of structure to any coronal appearance 
which was on the plate. Besides, any diminution of the transparency of 
. the solution, by the presence of minute particles would produce scattered 
light on the plate. 
I then tried a solution of iodine in carbon disulphide, but the same 
inconvenience presented itself.. Very soon under the sun’s light the 
solution was found by examination with a lens to show signs of com- 
mencing decomposition. 
Even when the solution was sensibly clear, there was some disadvantage 
from unavoidable imperfection of polish of the surfaces of the plates which 
reveals itself under the strong light in which they are placed. If, however, 
the violet (pot) glass which I used at first could be obtained annealed and 
free from the imperfections usually present in it, it would serve most 
usefully as a selective screen. 
For these reasons, after some months’ work, I decided to give up the 
use of absorbing media, and I came to the conclusion that the advantages 
they present, which are doubtless considerable, are more than balanced by 
the possible false appearances which they might give rise to if the solu- 
tions were not in a condition of perfect transparency. 
As, for the reasons stated above, it seemed desirable to avoid placing 
media of any kind before the sensitive surface, the selective power upon 
the light had to be sought in the nature of the sensitive surface itself. 
The suggestion of staining the film presented itself, but after consultation 
with Captain Abney, I decided to try an emulsion containing silver 
chloride only. Captain Abney kindly prepared some silver chloride 
emulsion fer me, and the plates were developed with a solution of ferrous- 
citro-oxalate. 
The silver chloride film, according to Captain Abney, is strongly 
sensitive to light from h to H, and hardly at all beyond H. 
Since the middle of July these plates have been used as well as the 
ordinary silver bromide gelatine plates. A comparison of the two kinds 
of plates, when used under similar conditions, shows a decided advantage 
for this work in favour of the silver chloride. 
All the plates were backed with a solution of asphaltum in benzole. 
For the purpose of screening the sensitive surface from the intensely 
bright image of the sun, small circular disks of thin brass were turned 
abont =}, inch larger in diameter than the sun’s image. The brass disk was 
held close before the sensitive surface by a fine metal arm when the sun 
was taken in the middle of the field, and attached to the inner edge of a 
circular diaphragm when the sun’s image was placed towards the side of 
the field. : 
A comparison of photographs taken under similar conditions with 
and without the disk showed less advantage in favour of the disk than was 
anticipated. Indeed, it may be that, with the short exposures given, the 
scattered light, which comes upon the plate when the sun’s image falls 
directly on the sensitive surface, may be favourable to the setting up of 
the photographic action by the comparatively feeble coronal light. 
In consequence of the number of diaphragms which it was found 
