Page:Proceedings of the Royal Society of London Vol 69.djvu/240

224 12. Dallmeyer Telephotographic Camera.

The double camera used by Professor Turner at Sahdol in 1898 and at Algiers in 1900 was used on the present occasion in the modified form adopted at Algiers. One of the 7 x 7-inch tubes contained the photoheliograph objective No. 2, of 4-inch aperture and 5-feet focal length, with a Dallmeyer secondary magnifier of 7^ inches focus, placed 5 inches within the focus, giving an image of the sun T43 inches in diameter on plates 6^ x 6^ inches.

The method adopted for focussing was first to focus on trees on a distant hill top, viewed through blue glass, and then to use the colli- mating process, adopted in many recent eclipses. A plane mirror set perpendicular to the axis of the camera reflects rays emerging from the objective back again through the objective ; the image of objects in the focal plane is thus formed in the focal plane. The mirror used on the present occasion was the 6-inch kindly lent to me by Dr. Common, and a negative, specially prepared with an image of a finely graduated scale in white on a black ground, was used as the object to be focussed on a sensitive plate, which was put in the same plane with it. Four trials gave sufficient data to set the focus accurately.

The results obtained in the eclipse are as satisfactory as the lenses will give, and it seems right to suggest that some other lenses be used, if it is decided to take this kind of photograph of the corona in future eclipses.

The programme of exposures with this camera was of necessity the same as that adopted for the polariscopic images referred to in 13, p. 227, for the exposures were made by means of the one shutter for both cameras. The six slides were filled with Ilford Empress plates, and all six were to be exposed during totality if circumstances allowed. It was intended, if the sky was clear, that the exposures should include the series, 2 s, 4 s , 8 s , 16 s , 32 s , and another long one (120 s ) with diminished aperture; and in case of cloud the long exposure was, if necessary, to be sacrificed altogether, and the ex- posures in geometrical series were to be secured, if possible, in clear intervals. The order of the exposures was to be :

1. 8 seconds, followed by an interval in which the Savart camera

would be set by me after visual observations of the atmo- spheric polarisation had been made.

2. 16 seconds.

3. 32 seconds, after which Dr. Wallace was to put a cap on the

lens to diminish its aperture.

4. Long exposure, to be sacrificed if necessary ; after this the cap

Avas to be removed again from the lens.

5. 4 seconds.

6. 2 seconds.