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.