Popular Science Monthly 



905 



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 



