He paused to pull up photographs on the screen.
The photographs led to no more definite result than the observations of contacts, except perhaps those taken by the Americans, who had adopted a more complete system than the Europeans; but even these were by no means satisfactory.
There are farther inconveniences in the use of such a telescope, viz., that the image undergoes a diurnal rotation about the axis of the horizontal telescope, so that, unless the sensitive plate is also rotated by clockwork, it is impossible to obtain sharp photographs with any but instantaneous exposures.
Recently the proper motions of faint stars have been determined by comparing photographs of the same region of the sky, taken with an interval of a number of years.
Usually the changes are such that it is impossible even to recognize the formations in successive photographs.
