a backward and forward periodical pulsation due to orbital motion.  As the pair whirled round their common center of gravity, the bright star was sometimes advancing, at others receding.  They revolved in about four days, each star moving with a velocity of about 56 miles a second in an orbit probably nearly circular, and possessed a combined mass of rather more than two and one-half times that of the sun.  Taking the most probable value for the star’s parallax, the greatest angular separation of the stars would be far too small to be detected with the most powerful telescopes.

THE VALUE OF PHOTOGRAPHY.

Referring to the new and great power which modern photography had put into the hands of the astronomer, the president said that the modern silver bromide gelatine plate, except for its grained texture, met his needs at all points.  It possessed extreme sensitiveness, it was always ready for use, it could be placed in any position, it could be exposed for hours, lastly it did not need immediate development, and for this reason could be exposed again to the same object on succeeding nights, so as to make up by several installments, as the weather might permit, the total time of exposure which was deemed necessary.  Without the assistance of photography, however greatly the resources of genius might overcome the optical and mechanical difficulties of constructing large telescopes, the astronomer would have to depend in the last resource upon his eye.  Now, we could not by the force of continued looking bring into view an object too feebly luminous to be seen at the first and keenest moment of vision.  But the feeblest light which fell upon the plate was not lost, but taken in and stored up continuously.  Each hour the plate gathered up 3,600 times the light energy which it received during the first second.  It was by this power of accumulation that the photographic plate might be said to increase, almost without limit, though not in separating power, the optical means at the disposal of the astronomer for the discovery or the observation of faint objects.

TWO EXAMPLES.

Two principal directions might be pointed out in which photography was of great service to the astronomer.  It enabled him within the comparatively short time of a single exposure to secure permanently with great exactness the relative positions of hundreds or even of thousands of stars, or the minute features of nebulae or other objects, or the phenomena of a passing eclipse, a task which by means of the eye and hand could only be accomplished, if done at all, after a very great expenditure of time and labor.  Photography put it in the power of the astronomer to accomplish in the short span of his own life, and so enter into their fruition, great works which otherwise must have been passed on by him as a heritage of labor to succeeding generations.  The second great service which photography rendered was not simply an aid to the powers the astronomer already possessed.  On the contrary, the plate, by recording light waves which were both too small and too large to excite vision in the eye, brought him into a new region of knowledge, such as the infra-red and the ultra-violet parts of the spectrum, which must have remained forever unknown but for artificial help.