without an Eclipse. “ 29 
up the use of absorbing media, and I came to the conclusion 
that the advantages they present, which are doubtless consider- 
able, are more than balanced by the possible false appearances. 
which they might give rise to if the solutions were not ina 
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 
for me, and the plates were developed with asolution of fer- 
rous-citro-oxalate. The silver chloride film, according to Cap- 
tain Abney, is strongly sensitive to light from % to H, an 
hardly at all beyond H. Since the middle of July these plates 
ave 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 favor 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 discs of thin 
rass_were turned about ;;th of an inch larger in diameter 
than the sun’s image. The brass dise 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 toward the 
side of the field. A comparison of photographs taken under 
Similar conditions with and without the disc showed less ad- 
vantage in favor of the disc 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 favorable to the setting up 0 
aa Photographie action by the comparatively-feeble corona 
8 
In consequence of the number of diaphragms which it was 
found desirable to introduce into the apparatus for the purpose 
of preventing any light but that from the sun and the sky im- 
t The moving shutter being placed very near the sensitive sur- 
ace, and practically in the focal plane, could not give rise to 
effects of diffraction upon the plate; besides, the opening | 
