Page:Proceedings of the Royal Society of London Vol 69.djvu/271

Rh (1.) Plwtographs on the 8-inch Scale.

These were taken by Mr. T. F. Claxton with the photoheliograph of the Royal Alfred Observatory, Mauritius, the object-glass of which is 4 inches in aperture and 5 feet in focal length. It gives an image of the Sun nearly 8 inches in diameter, being fitted with a secondary magnifier, enlarging the image in the primary focus 13| times. The total length of the photoheliograph was 9| feet ; its equivalent focal length 69 feet. The camera was constructed to carry plates 10 inches square, and it was proposed to expose six plates during totality for photographs of the corona, and as many as possible during the partial phase for determination of the Moon's place, and of the relative diameters of the Sun and Moon. Only three plates were exposed during totality, the handle of the shutter of the fourth plate-carrier breaking short off when the shutter was opened. The plate-carrier was thus locked in the camera, and it was not possible to release it until the total phase was over. The three plates taken were exposed as below, the exposures being given by Mr. Rowden at the object- glass of the instrument at the sound of the signal bells.

No.

From

To

Duration of

Plate.

exposure.

eec.

1

Bell 3

Bell 5

20

Edwards' Medium.

2

6

,. 8

20

Imperial Ordinary.

3

8

ii 11

20

Special Kapid.

After totality the clock of the ccelostat was stopped, and three photographs of the Sun were taken on the same plate for orientation. The true local times of the first and last of these were :

h.

m. 15 16

s.

20-1 5-2

The time of the second exposure was not recorded. The plate was an "ordinary lantern" by Cadett and Neale. The aperture of the photoheliograph was reduced to 1 inches, and the exposure was given by the drop-slit in the primary focus.

Two sets of " Abney squares " were printed upon No. 3 on June 13, before the plate was developed. Both sets were printed by the light of a Sugg's Standard candle ; the one being exposed for 4 seconds, the other for 64 seconds, the candle being at a distance of 10 feet in both cases.

The corona is seen only on the E. side of the Sun, as the ima