130 THE FUNCTION OF LARGE TELESCOPES. 



cal benefit the astronomer enjoys from the special properties of large 

 lenses which have just been enumerated. 



Like other scientific men, astronomers who expect to accomplish 

 much of importance at the present day find it necessary to specialize, 

 and to devote their attention to certain classes of work in which long 

 study and experience have given them particular skill. Thus it is that 

 to some astronomers certain of the advantages of a large telescope 

 appeal much more strongly than do others. In fact, in order to derive 

 the best results from the use of the instrument it is necessary to have 

 observations made with it by men who are capable of bringing out its 

 best qualities in various kinds of investigation. Thus the first-men- 

 tioned property of rendering visible faint objects should be utilized by 

 an astronomer who has gained much experience in searching for and 

 measuring objects at the very limit of vision. One who has not given 

 special attention to this class of work would be surprised to see in a 

 large telescope certain of the faint stars or satellites of whose discov- 

 ery he may have read. When the fifth satellite of Jupiter was discov- 

 ered at the Lick Observatory by Professor Barnard, in 1892, claims 

 were put forward by certain amateur astronomers who possessed small 

 telescopes that they themselves were entitled to the honor of the dis- 

 covery, for they had seen the satellite long before. Such claims might 

 be taken in earnest by one unfamiliar with the instruments employed 

 by the respective observers. But it is only necessary to examine this 

 minute object with a 36-inch or a 40-inch telescope in order to appre- 

 ciate the great merit of the discovery and the absurdity of such claims 

 as have been mentioned. The tiny satellite is so faint that hitherto it 

 has been seen with very few telescopes, all of them having large aper- 

 tures. In its rapid motion close to the surface of the great planet it is 

 completely invisible to an eye unprotected from the brilliant light of 

 Jupiter. Even the close approach of one of the other satellites is suf- 

 ficient to cause it to disappear. In measuring the satellite Professor 

 Barnard finds it necessary to reduce the light of Jupiter with a piece 

 of 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 recent measures of this satellite with the Yerkes 

 telescope have shown that his original determination of the time of its 

 revolution in its orbit, made five years ago at Mount Hamilton, was not 

 in error more than 0.03 second. It was found that the time of elonga- 

 tion differed less than half a minute 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 Prof. 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 Naval Observatory. 



