There is, however, a still better safeguard—the use of photography. The transit will record itself more minutely and more accurately than any ordinary observations for measurement could do. Various plans will be used. One proposed is to have one hundred and eighty prepared and highly sensitive plates along the circumference of a suitable wheel made to revolve regularly by clock‐work. During three minutes, these plates come, one every second, successively into position to receive and record the images of the transit, as the planet for those three minutes is entering on the sun. Other plates, at stated and accurately measured intervals of time, will similarly record its regular progress across the sun; and another wheel, with one hundred and eighty other plates, will record the successive changes each second for the three minutes occupied by its exit over the sun’s border. These are all, of course, negatives on glass. From them any number of impressions can be taken, in the usual way, for general distribution among the scientists. In order that such impressions may still serve for the finest measurements, despite of any variations of expansion, contraction, or warping which the atmospheric changes may produce, a system of fine, spider‐web lines is placed inside the telescope, producing on the photograph itself a network of fine lines, some running north and south, others crossing them east and west. These lines are at equal distances apart, and serve admirably for measuring the position of the planet on the solar face. If the photographic sheet should become quite distorted, these lines would show it; for they would of course follow the distortion, and yet, after that distortion, they would still guide us to accurate measurements. It is hoped that this means and the many other photographic devices to be used will secure a degree of accuracy far beyond what Halley anticipated and would have been satisfied with.