Several new departures in eclipse photography were introduced. Chief amongst these was the remarkable apparatus by means of which no less than twenty-three objectives and two mirrors were accurately pointed at the sun and caused to follow it by one large clock. A large duplex polar axis (the old English form as used for the 12.5 inch reflector at Greenwich) was mounted on solidly constructed stone piers and very carefully adjusted. This axis is constructed of 6–in. wrought-iron tubing, the total weight being about 2000 lbs. In it the cameras were fixed by set screws, the optic axis of the instruments being adjusted parallel to each other, and at an angle equal to the south polar distance of the sun at the time of totality. The carefully regulated and very powerful clockwork attached to the instrument caused the polar axis to rotate, and thus the whole battery of instruments followed the sun. Each lens was fitted with a pneumatic shutter regulated to give the required exposure in each case. The cameras themselves were enclosed in a dark-room, the lenses only being exposed to the sun, so that dark slides were not required, the plates being held on open rotating frames, these frames being rotated at the proper time by pneumatic arrangements. When the cameras were once pointed, and the clock driving properly, all the operations of exposure and changing of plates were performed without personal superintendence by means of the pneumatic apparatus, and a chronograph attached to the valve system of this apparatus recorded the exact time at which each exposure was begun and ended.
It is to be regretted that this ingenious and elaborate apparatus did not have a satisfactory trial, owing to the dense clouds; but Professor Todd assures us that he was thoroughly satisfied with the success of the pneumatic movements during the three minutes ten seconds he brought it into operation at the time of totality.
It is not improbable that (in spite of the great strength and weight of the axis and the solidity of the supporting piers) with this plan of fixing a large number of cameras and spectroscope on one polar axis, the constant opening and shutting of shutters, and the changing of the plates, may produce so much shake that none of the long exposure photographs will be satisfactory. This, of course, can only be ascertained by the use of the instrument on the corona, and several years must elapse before the trial can be made.