Page:Proceedings of the Royal Society of London Vol 69.djvu/272

256 somewhat out of the centre of the plate in all three of the photo- graphs ; and owing partly to shake in the instrument, and partly to the excessive atmospheric disturbance, which under the high magni- fication employed was very apparent, the definition is poor in each case.

No. 1. The corona is shown to a height of about 1 minute of arc. The image is faint.

No. 2. The corona is shown to a height of about 3 minutes of arc. The image is fairly dense.

No. 3. The corona is shown to a height of about 2 minutes of arc. The image is faint and flat.

The Mauritius photoheliograph was focussed by repeated photographs of the Sun, taken during the fortnight preceding the eclipse.

(2.) Photographs on the 2'i-inch Scale.

These were taken with the object-glass of the Dallmeyer photo- heliograph No. 5, belonging to the Royal Observatory, Greenwich and which, like that of the Mauritius photoheliograph, had an aperture of 4 inches and a focal length of 5 feet used in connection with a concave telephoto lens by Dallmeyer, of 3 inches aperture and 12 inches focal length. The instrument thus constructed has been referred to for convenience as the " Greenwich coronagraph," and was about 7^ feet in actual length, with an equivalent focal length of nearly 21 feet. It was furnished with ten plate-holders to take 12 by 10-inch plates, and it was proposed to take seven photographs during totality.

These photographs were taken by myself, Colour-Sergeant-Major Wade exposing at the object-glass of the instrument by the signal bells as follows :

Duration

No.

From

To

of

Plate.

exposure.

sees.

1

Bell 2

Bell 3

10

Wratten and Wainwright Instantaneous.

2

4

6

20

Ilt'ord Empress.

3

7

11

40

Imperial Special Rapid.

4

12

15

30

,, Sovereign.

5

16

18

20

,, Fine Grain Ordinary.

6

19

20

10

Special Rapid.

7

21

4

,, Ordinary.

After the ground glass had been inserted for the observation of third contact, and had been removed again, the aperture was diminished to half an inch, and the clock of the coelostat stopped and three photo