262 The Achromatic Telescope — Occiiltation. 



will be formed free from the inconvenience of coloured fringes, 

 and thus we shall have constructed the object-glass of a tele- 

 scope in which the imperfection arising from dispersion will 

 exist no longer. 



So far well. One defect has been exterminated ; but what 

 meanwhile has become of the other — the spherical aberration ? 

 It most fortunately happens that this error does not depend 

 wholly upon the focal length, but (within certain limits) upon the 

 form of the lens, and its amount changes with every change in 

 the many combinations of curves corresponding to the same 

 focal length. The aberration, for instance, of a lens of equal 

 convexity on each face will be different from that of a plano- 

 convex one of the same focus ; and the latter again will be 

 very different according as its plane or convex side is turned 

 towards the incident rays; and the same holds good with 

 concave lenses. Thus the circumstance of being obliged to 

 combine two lenses to neutralize the error of colour renders it, 

 remarkably enough, practicable to correct the spherical error 

 at the same time, for we have only to make such a choice of 

 curves as, without altering the focal lengths, will produce equal 

 and opposite spherical aberrations in the two lenses, to reduce 

 the defect to a minimum. And in this way a focal image will 

 at last be produced free from both the imperfections so in- 

 jurious to the performance of the ancient refractor, and our 

 telescopes, with equal aperture, will not only admit of a won- 

 derful reduction in length, but be actually more efficient, from 

 the destruction of those errors which had only been palliated 

 at the cost of so much inconvenience. Such is the beautiful 

 theory of the achromatic telescope. How far it has failed in 

 answering the full extent of expectation, with the causes and 

 attempted remedies of that failure, will form the subject of 

 some remarks upon another occasion. 



OCCDLTATION. 



There will be only one occiiltation before midnight during 

 the present month, on the 19th, when k Cancri, 5 mag., will 

 disappear from llh. 27m. to 12h. 16m. 



