iS M. Arago on Double Stars. 



ges of two contiguous points of the object necessarily encroach 

 upon one another ; their rays are confounded together, and it 

 will be impossible for the ocular lens to dissipate this confusion. 

 The office which it alone fulfils is to magnify ; it magnifies every 

 thing in the image, the defects as well as the rest. The tele- 

 scope, we mean the united lenses, cannot then represent the ob- 

 jects well defined. 



This defect of distinctness exists^ in different degrees, in tele- 

 scopes, according as the artist has succeeded in giving to the two 

 faces of the object-glass a regular curvature, more or less in ap- 

 proximation to the geometric form, which theory has established 

 as most suitable, and towards which the optician is always ap- 

 proximating, but which still always remains an abstraction. It 

 often happens, that with a single glance, whatever be the object 

 at which we look, we can know if the object-glass has been suc- 

 cessfully finished ; but this is not always the case. Astronomers 

 themselves, when called upon to compare two telescopes, how- 

 ever experienced they may be, sometimes feel difficulties, 

 if they have only examined large bodies, such as Venus, Jupi- 

 ter, Saturn, and Mars. In these cases, the double stars put an 

 end to all dubiety. 



It is proved that the stars have not sensible angular diame- 

 ters. Those which they appear to have, depend, for the most 

 part, on the imperfections of our instruments, and also on 

 certain defects, certain aberrations of the eye itself. The more 

 a star appears small, every thing being equal, as it regards the 

 diameter of the object-glass, the magnifying power employed, 

 and the brilliancy of the star observed, the more perfect is the 

 telescope. But the best means of judging whether the stars 

 appear very small, if points, such as these, are represented at 

 the focus by simple points, is evidently, to examine stars which 

 are excessively near to each other, and to observe if their images 

 are confounded, if they encroach upon one another, or other- 

 wise, if they be perceived to be distinctly separated. We sub- 

 join a list, from the known double stars, of a certain number of 

 those in which the best glasses alone, furnished with strong 

 magnifiers, succeed in effecting a separation. 



