The object of my expedition to Kalaa-es-Senam, Tunisia, was to obtain a series of photographs from which might be determined the distribution of light in the corona. In designing my apparatus, I was led by two considerations: (1) the photographs had to be taken automatically, as I had to work without assistance, (2) standardising of the photographs was to be avoided. All the photographs were therefore taken on the two halves of a whole plate placed end to end and developed in the same tray during the same time. The automatic apparatus gives 10 exposures, and it is governed electrically by a pendulum clock. I employed two cameras, one with a Cooke triple achromatic 3½ lens of inches aperture and 58·5 inches focal length, which belongs to the Glasgow spectrograph, the other with a Ross portrait lens of 2 inches aperture and 12 inches focal length. The pictures obtained with the larger camera are so much superior to the small size ones of the portrait lens that I have not made use of the latter in this paper. The cameras were fed by a cœlostat of 8 inches aperture, which had been kindly lent to me by the Royal Dublin Society. In front of the two object-glasses, and about an inch from them, a rotating shutter was mounted which served both cameras. The rotating shutter has four oblong apertures, 90 degrees apart (its back view is shown at D
2
, fig. 1); it is rotated by clockwork driven by a spring, and its motion is governed by the armature of an electro-magnet (
f
). When the armature is attracted, the shutter rotates through about 45 degrees until it presses against one of the four stops
d
and brings an opening opposite the object-glasses, and when the armature is released the shutter turns again 45 degrees, as far as one of the stops
c
, and shuts off the light. The contacts are made by a pendulum clock, and they are so devised that make or break can occur only when the pendulum is at or near its position of rest.
Boyle Medal of the Royal Dublin Society: Prof. J. H. J. Poole
