M. Arago 07i Double Stars. 37 



for a time at least, may occupy the attention of the most fervid 

 imagination. 



8. The double stars have become a means whereby we m^yjitdge of the 

 excellence of Telescopes. 



The distinffuishiijff of the stars is for those astronomers who 

 may be called upon to pronounce upon the excellency of tele- 

 scopes, whether of a more common, or of the most superior kind 

 — a touchstone more sensible and precise, in certain respects, 

 than the observation of the disc of planets has hitherto been. 

 The expressions that the glass distinguishes well, — that with it 

 we distmctly see the belts of Jupiter and of Saturn, — that the 

 spots of Mars are clearly perceived, &c. &c. are vague, and pos- 

 sess a different value, as they are pronounced by an astronomer 

 more or less accustomed to use powerful and well-made instru- 

 ments. These expressions, whoever employs them, always im- 

 ply, in the mind of the speaker, a confused idea of comparison. 

 But if it be said, that with a magnifier of 200 times, for ex- 

 ample, a glass completely separates the two stars, now so near 

 each other, which together form <r of the Crown, there is sup- 

 plied, to any one who tries a similar experiment, the means of 

 ascertaining, without any hesitation, if his instrument be infe- 

 rior to the other. If we, in a word, recall to recollection the 

 fundamental principle of every telescope, the advantages of this 

 kind of test will at once become evident. 



A telescope is composed of two glass lenses. The one large, 

 and turned towards the object, is called the object-glass ; the 

 other, quite small, and placed near the eye, is denominated the 

 eye-glass. The former lens forms, in a certain part more or less 

 distant from its surface, called the focus, an aerial image, — a 

 true picture of all the objects in view. It is this image — this 

 picture, which is enlarged by the help of the magnifying eye- 

 glass, just as if it were a natural object. 



When the focal picture is distinct, — when the rays which, 

 proceeding from a point of the object, are again concentrated in 

 a single point in the image, the observation then made with the 

 eye-glass gives results most satisfying. If, on the contrary, 

 the rays which emanated from a point do not reunite at the fo- 

 cus in a single point ; if they there form a little circle, the ima- 



