in the centre f inch in diameter, through which a spindle, mounted on the back of the plate carrier, was passed. On this spindle the screen was fixed in the dark room after the plate had been put in the carrier.'
Occulting Shutter for Photographs of Corona by Burckhalter's Method.
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The figures against the dotted lines show the effective exposures at different distances from the Sun's centre.
During the exposure, the observer turned a small handle about once a second, which, by a suitable train of wheels, caused the screen to turn about eight times a second. The apparatus appears to work perfectly smoothly, as the limb of the Moon is sharply defined on the photographs taken.
The eight slides were exposed as below, the exposures being made at the object glass by Dr. Nimmo :
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Exposure.
Begin-
End.
Dura-
Plate.
ning.
tion.
i
m. s.
m. s.
sees.
in.
1
30
40
10
Mawson Photo-mechanical. 12 x 10
2
1
1 10
10
Edwards' Ordinary. 12 x 10
3
1 20
1 40
20
Ilford Empress. 15 x 15
4
2
2 40
40
15 x 15
5
2 50
3 30
40
Barnet Rocket. 15 x 15
6
3 40
3 45
5
Imperial Ordinary. 12 x 10
7
4 5
4 15
10
., Sovereign 15 x 15
8
4 35
4 55
20
Wratten and Wainwright Ordinary. 12 x 10
A ninth plate was taken at 1 o'clock, local time, on which two exposures were made (the driving clock being stopped for two minutes between) for determination of the orientation of the plates.
