Popular Science Monthly
��entering into this problem, the task was not easy, but it was finally accomplished by two English astronomers, and the predicted positions were published far in advance of the expected appearance of the comet, so as to enable the astronomers to secure its image on a photographic plate as early as possible. The accom- panying illustration shows it as found on negatives taken at the Yerkes Ob- servatory in 1909. In appearance it does not differ markedly from the stars on the plate. It was its change in position which led to its detection.
Calculating the orbit of Halley's comet is one of the most difficult mathematical problems in all astronomy. As it rushes through the heavens the comet is attracted by one planet and repelled by another. Constantly subjected to vari- ous forces difficult to appraise, the prediction of its return is well worth a prize. The search for comets is so persistent that most of them are discovered while still faint tele- scopic objects. One, however, seen during the past few years, the great comet of Januar>% 19 10, came toward us from be- hind the sun in such a way that when first seen it was already a very bright object. Though it was a morning comet, and visi- ble only in southern latitudes, it was so conspicuous that it was discovered inde- pendently by several people. Our photo- graph of it shows it as seen two weeks later at the Lowell Obser\ ator>'. Flagstaff, Arizona, by which time it had swung around to the other side of the sun.
The comet-seeker finds his counter- part in the satellite hunter. Many of the planets have moons. Galileo himself discovered four of Jupiter's with the little telescojjc that hedevised. In 1877, shortly after the 26-inch in- strument of the United States Naval Observator>' was put in place. Profes- sor Asaph Hall made a long and per- sistent search for satellites of Mars. He was just on the point of giving up, continuing one night longer only be- cause his wife urged him, when to their great joy the discovery was
���Dr. William R. Brooks, Director of Smith Obser- vatory and Professor of Astronomy of two colleges
��made. These two tiny bodies, being very close to their primary, defied detection for a long time.
Again, in 1892, while the great thirty-six-inch telescope of the Lick Observatory- was young to the astronomical world. Professor Barnard deter- mined to find out if the planet Jupiter had any other satellites than the four seen by Galileo with the first telescope ever pointed toward the heavens. He was re- warded by the discovery of the fifth satellite.
The Aid the Camera Gives
There are limits to the possibilities of even the most powerful telescope; but there are no limits to the things that can be discovered in the universe. Artificial eyes had to be improved. That was done, not so much by making tele- scopes bigger, as by using them in connection with the photo- graphic plate. In the back of your eye is a very sensitive surface called the retina upon which images of the outer world are thrown. You really see with
your retina. The photo- graphic plate is al- so a kind of retina. By com- bining it th the
���Intra-mercurial cameras mounted at Cartright, Labrador, at the time of the total eclipse of August 30, 1905, by the Crocker eclipse expedition