May 13, 1898.] 



SGIENGR 



657 



■ smoked mica, through which the planet is 

 still clearly visible and easily measurable, 

 though not annoying to the eye. Without 

 an instrument like the Lick telescope the 

 fifth satellite of Jupiter would never have 

 been known. It may be interesting to 

 mention here that Professor Barnard's re- 

 cent measures of this satellite with the 

 Yerkes telescope have shown that his orig- 

 inal determination of the time of its revo- 

 lution in its orbit, made five years ago at 

 Mt. Hamilton, was not in error more than 

 0.03 seconds. It was found that the time 

 of elongation differed less than half a min- 

 ute from the time predicted in the Nautical 

 Almanac. The period is now known within 

 a few thousandths of a second. In this 

 connection also it is well to add that Pro- 

 fessor Asaph Hall's discovery in 1877 of the 

 two small satellites of Mars was directly 

 due to the advantage given him by the 

 large aperture of the 26- inch telescope at 

 the United States Kaval Observatory. 



Such small members of the solar system 

 are by no means the only feebly luminous 

 objects which great telescopes have brought 

 to light. Faint stars in the close proximity 

 of bright ones are usually beyond the reach 

 of small telescopes. Thus the companion 

 of Sirius was not seen until 1862, when the 

 late Alvan G. Clark encountered it in his 

 tests of the 18-inch objective now at the 

 Dearborn Observatory, which was the larg- 

 est glass that had been constructed up to 

 that time. The small companion to Pro- 

 cyon, discovered not long ago by Professor 

 Schaeberle with the Lick telescope, is 

 another object of the same type. These 

 are conspicuous examples of that great class 

 of objects known as double stars, which 

 consist of two stars revolving about their 

 common center of gravity. From the third 

 advantage of large instruments to which 

 reference has already been made, it will be 

 seen that they are peculiarly adapted for 

 the investigation of these binary systems. 



not only because of their power to show 

 faint objects in the neighborhood of brighter 

 ones, but also on account of their capacity 

 to separate two closely adjacent stars which 

 in a smaller instrument would be seen as 

 one. Thanks to this property, many inter- 

 esting binary systems whose components 

 are exceedingly close together have been 

 found by Professor Burnham with the Lick 

 telescope, and, although he has devoted no 

 special attention to a search for such ob- 

 jects, Professor Barnard has already en- 

 countered several of them in his work with 

 the Yerkes refractor. From what the 

 spectroscope has taught us of binary sys- 

 tems, we have every reason to believe that 

 telescopes may go on increasing in aperture 

 almost indefinitely without ever arriving at 

 the possibility of separating into their com- 

 ponent parts all existing double stars. As 

 has been stated, the Yerkes telescope can 

 show as distinct objects stars which are no 

 further apart than 0."12 of arc, and on ac- 

 count of the elongation of the image a 

 double star whose components are only 

 0."1 apart can be distinguished from a sin- 

 gle star. But there undoubtedly exist 

 stars far closer together than this, some of 

 which can be separated by an aperture of 

 not less than forty feet. 



There has been much discussion in recent 

 years regarding the relative advantage of 

 large and small telescopes for observations 

 of the markings on planets. I do not 

 propose to enter into the details of this 

 discussion, partly because my own inves- 

 tigations are primarily concerned with ob- 

 servations of another nature, and thus have 

 not especially qualified me to form an 

 opinion on this point, and partly on ac- 

 count of the fact that additional arguments 

 in favor of large instruments would serve 

 little purpose. It seems to me only neces- 

 sary for an unprejudiced person to examine 

 a planet first with a small telescope of from 

 five to fifteen inches aperture, and then to 



