NOTES ON PLATE EXPOSURES. 187 
photographs were taken by one instrument, the exposures 
were spaced on either side of these values. 
The sixteen foot Cecil Darley telescope described under 
(1), and operating at f 64, was used for six exposures, the 
times being 3, 6, 12, 24,48, and 12 seconds. It will be 
noticed that the “doubling up”’ principle was employed— 
modern plates have such a wide latitude that any smaller 
change of exposure ratio than two is unnecessary. The 
coelostat worked excellently, and the six plates were suc- 
cessfully exposed. 
All development was carried out at the University, 
the developer employed being the laboratory standard 
Metol-Quinol.* A six tray method was employed, the third 
tray being a 1:32 mixture (normal). The first tray was 
1:128 (very weak), the second 1:64 (weak), the fourth 1:16 
(strong). The fifth tray contained a 1:1000 solution of 
potassium bromide, and the sixth tray pure water. The 
plates were developed singly, after the backing had been 
removed by washing. The dark room illumination was a 
No. 2 Wratten Safelight for the special sensitive plates, 
and a No.3 Wratten Safelight for the panchromatic plates. 
Placed first in No. 1 tray, the time of first appearance of 
image was noted. Plates in which the image showed up 
in from 60 to 90 seconds were considered normal, and 
transferred to No. 3 tray, development taking normally a 
further five minutes. Plates on which the image showed 
up in less than 60 seconds were transferred to No. 5 tray 
(KBr) for a time from 10 to 90 seconds, depending on the 
degree of overexposure, then to No. 6 tray where develop- 
ment took about an hour. Those plates on which the 
image took longer than 90 seconds to appear were trans- 
ferred to No. 4 bath for two minutes, then to No. 6 tray 
for about an hour, or till general fog began. 

* Booth, Aust. Photo. Review, Vol. 29, p.16. British Jouinal of 
Photography, Vol. 69, p. 168. 
