440 JOURNAL, R.A.S. (CEYLON). [VOL. XVI II. 
employed. Finally, for obtaining the brilliant yellow colour 
value positive a blue violet colour screen is used. In 
their practical application these several colour screens can 
be adapted to fit on in front of the camera lens or to be 
interposed at the back immediately in front of the exposed 
plate. 
The taking of negatives through these variously coloured 
screens necessarily involves considerably longer exposures 
tlian obtains in ordinary monochrome photography. This 
is more especially noteworthy of the one obtained through 
the red screen for which an exposure is given approxi- 
mating four or five times the time duration associated 
with the green screen and ten or twelve times that re- 
quired when exposing through the blue violet screen. Thus 
when taking negatives of ordinary subjects in full tropical 
sunlight such as that of Ceylon, as I have been doing lately, 
I have found that wifch the lens well stopped down to F. 96 
an exposure of two and a half minutes with the red screen, 
forty-five seconds with the green, and fifteen seconds with 
the blue screen represent the most satisfactory time ratios. 
These time ratios are however subject to slight modifications 
with respect to every ne w batch of plates used, and for which 
it is desirable to make one or more tentative exposures. A 
sure method of arriving at the correct time ratio of any given 
batch of plates is by including a pure white object in 
the photographic field, and which should develop with equal 
density on each of the three plates exposed. 
Subjects photographed in the shade and more especially in 
a dull or murky atmosphere such as prevails during an Eng- 
lish winter require a considerably longer exposure than the 
periods just enumerated. Exposures of five minutes, one and 
a half minute, and thirty seconds, respectively, represent the 
ordinary duratiou of the time exposures employed for fche 
majority of the examples exhibited on this occasion. In 
some instances, however, such as that of the portrait in half- 
plate size, no less than fifteen minutes were occupied in the 
exposure under the red screen, five minutes with the green 
