Double Stars. 151 



most part foreshortened in perspective ; and hence circular 

 orbits (if there be any) will usually be projected on the sky as 

 ellipses, and ellipses will be thinned off till in some cases they 

 are seen edgeways as straight lines, and the stars will appear, 

 twice in every revolution, to close up into one, at least, with 

 such optical means as are at our command. A true occultation 

 is, however, probably extremely infrequent, as we have reason 

 to think that the real discs of the stars are too minute to be 

 perceived — the merest points conceivable. They have never 

 yet been seen by mortal eye. The circular appearance which, 

 in a fine state of atmosphere, a star exhibits in a telescope, is 

 not its real image, but what Sir W. Herschel termed a "spurious 

 disc/'' arising partly from peculiarities in the original constitu- 

 tion of light, and partly from unavoidable imperfection in the 

 instrument. With low magnifying powers, indeed, this little 

 disc is not to be distinguished from a point, but these are in 

 general inadequate to the purposes of sidereal astronomy ; as 

 we employ higher magnifiers, we shall find that the point will 

 expand into a little luminous circle, surrounded by one or two 

 faint rings, which are present in every good telescope; one 

 great test of goodness being the perfect circularity of the disc 

 and rings ; by increasing our aperture, we shall diminish the 

 proportional diameter of the disc, and hence it is that the same 

 magnifying power in a small telescope will fail in separating the 

 discs of close double stars, which are readily divided by a larger 

 one; but we have never yet succeeded, and have little hope 

 that we ever shall, in contracting the disc to so minute a point 

 as to be a true representation of the star. The progressive 

 diminution of the discs, as telescopes attain greater magnitude 

 and perfection, is at present but a remote approximation to the 

 real image, as is conclusively shown by the instantaneous dis- 

 appearance in occultation of a star even of the first magnitude, 

 behind the slowly advancing limb of the moon ; and hence it is 

 highly improbable that a real eclipse of one star by another has 

 ever been observed. In such a case, as Sir J. Herschel has 

 remarked, the fact would become evident to the naked eye, from 

 the diminution of the total amount of light emitted jointly by 

 the pair. 



The possessors of small telescopes will be glad to find that 

 a considerable number of interesting double stars, and even 

 binary systems will be within their reach. They will, of course, 

 not expect to distinguish the closer pairs, or to pick up the 

 miuuter comites ; nor will they succeed very well in the discri- 

 mination of colour, which becomes more distinct and full in pro- 

 portion to the quantity of light. But they may gain such a 

 glimpse into the interior of the temple as may give them some 

 slight idea of its grandeur and glory, and in these days, when 



