WRITTEN FOR THE MENTOR BY PAUL L. ANDERSON
ILLUSTRATION FOR THE MENTOR. VOL. 6, No. 12, SERIAL No. 160
COPYRIGHT, 1918, BY THE MENTOR ASSOCIATION, INC.

PHOTOGRAPHED AT YERKES OBSERVATORY, WILLIAMS BAY, WISCONSIN. OCTOBER 19, 1911

BROOKS’ COMET—Astronomical Photograph

PHOTOGRAPHY
Astronomical Photography

THREE

Photography has made possible many discoveries of tremendous importance in the realm of astronomy by revealing the existence of stars too faint—because of their small size or great distance—to be seen by the eye. This is one of the most conspicuous ways in which the sensitive plate has been an aid to the scientist. A device for carrying a photographic plate is attached to a telescope and the plate exposed to the image projected by the telescope for a prolonged period. This may, in fact, amount to several hours; exposures are sometimes partly completed one night and finished the next, a comparatively small area of the heavens being chosen for investigation at one time. On development of the plate the stars are counted and compared with existing charts of the area in question. Of course this method requires that the telescope move with the same angular velocity as that of the earth’s rotation, so that the image of each star may remain in precisely the same position on the plate during the entire time of exposure. Otherwise the star would be represented as a trail of light, the slightest variation in the speed of rotation being sufficient to cause blurring of the image. It is apparent that the clockwork employed for driving the telescope must be a marvel of accuracy.

The power which this method possesses of revealing hitherto undiscovered stars depends on a curious fact. If an observer looks into the eye-piece of a telescope he can discern only those heavenly bodies that send to the earth a certain minimum of light; but when a photographic plate is exposed for long periods there is a cumulative effect of light on the sensitive emulsion. That is, the long-continued impact of the light rays causes, little by little, a gradual change in the constitution of the sensitive silver salt. The action thus piles up, so to speak, and records light that is far below the visible minimum.