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SCIENCK 



[N. S. Vol. VII. No. 176. 



double star, the two components of which 

 are separated by a distance of 0".5 of arc, 

 it will be barely possible with a 10-inch 

 telescope to resolve the star into two points 

 of light just touching one another. If the 

 members of the pair are closer than this 

 they cannot be separated with a 10-inch 

 glass, no matter what magnifying power is 

 used. With a 40-inch telescope, on the 

 other hand, it is not only a simple matter 

 to separate stars 0".5 apart, but it is even 

 possible to distinguish as two points of 

 light the components of a double star of 

 only 0".12 separation. 



To sum up, then, we see that the prin- 

 cipal advantages of a 40-inch object-glass 

 as compared with one of 10 inches aperture 

 are : first, its power of giving much brighter 

 star images, and thus of rendering visible 

 faint stars which cannot be seen with the 

 smaller telescope; second, the fact that it 

 gives at its focus an image of any object, 

 other than a star, four times as large as the 

 image given by a lens of one-fourth its 

 aperture and focal length ; and third, its 

 capacity of rendering visible as separate 

 objects the components of very close double 

 stars or minute markings upon the surface 

 of a planet or satellite. Mention should be 

 made here of the fact that the large glass 

 assuredly has some disadvantages as com- 

 pared with the smaller one, particularly in 

 that it requires better atmospheric condi- 

 tions to bring out its full qualities. But I 

 think it will be seen from what follows that 

 these disadvantages are by no means suffi- 

 cient to offset the great advantages pos- 

 sessed by the larger instrument. Let us 

 now consider what practical benefit the 

 astronomer enjoys from the special proper- 

 ties of large lenses which have just been 

 enumerated. 



Like other scientific men, astronomers 

 who expect to accomplish much of impor- 

 tance 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 tele- 

 scope 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 inves- 

 tigation. Thus the first mentioned prop- 

 erty 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 at- 

 tention to this class of work would be sur- 

 prised 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 discovered 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 en- 

 titled to the honor of the discovery, for 

 they had seen the satellite long before. 

 Such claims might be taken in earnest by 

 one unfamiliar with the instruments em- 

 ployed 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 appreciate 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 apertures. In its rapid mo- 

 tion close to the surface of the great planet 

 it is completely invisible to an eye unpro- 

 tected from the brilliant light of Jupiter. 

 Even the close approach of one of the 

 other satellites is sufficient to cause it to 

 disappear. In measuring the satellite Pro- 

 fessor Barnard finds it necessary to re- 

 duce the light of Jupiter with a piece of 



