42 M. Fraimhofer on the Const ructioji 



be of an immense size to produce any effect; on which ac- 

 count the intensity of light that strikes the eye of the observer 

 is still very small. Nor is it possible entirely to rectify the 

 deviation of the rays occasioned by the spherical forms of the 

 reflecting surfaces of these telescopes ; — reasons which, with 

 various others, have rendered the mirror telescopes of little 

 value for mathematico-astronomical observations, and have 

 caused their rejection as meridional instruments, &c. 



The glass, on the other hand, allows all the rays to pass ; and 

 not only compensates in a telescope, made of flint- and crown- 

 glass, for the deviation of the rays on account of the refrangi- 

 bility of the colours, but this deviation may even be removed 

 on account of the spherical forms of the glass surfaces; by which 

 means the effect of achromatic telescopes is much more power- 

 ful than that of mirror telescopes. And it is for this reason, 

 and on account of their construction rendering them fit for 

 every species of observations, that at present almost all astro- 

 nomical observations are performed with achromatic tele- 

 scopes. 



Although the largest achromatic telescopes hitherto used 

 are small, compared with the largest mirror-telescopes, they 

 have in many respects produced more important results than 

 the latter. The best trial of telescopes is, as every one knows, 

 the observation of double stars ; in which, the effect of the 

 glass telescopes is decidedly greater than with the mirror tele- 

 scopes. Thus, for instance, M. Bessel, of Konigsberg, dis- 

 covered with an achromatic telescope made at this place, the 

 object-glass of which is but 48 lines, that the star ^ Bootis 

 (stated by Herschel to be a double star of the 4th class) be- 

 longs, at the same time, to the 1st class; since it has, besides 

 the principal star, another star near it, which Herschel did not 

 see. In the same manner several fixed stars observed before, 

 have only been found lately to be double stars when observed 

 through achromatic telescopes. 



It is a fact well known, that the effect of the telescope lies, 

 not in its length, but in the diameter of the object-glass; so 

 that for instance, among telescopes of a proportionably equal 

 perfection, that telescope whose object-glass is of twice the 

 magnitude of another will also be of double its strengtli. The 

 difficulties of making large achromatic telescopes proportion- 

 ably good with small ones, do not increase so much in the 

 proportion of the diameter of the object-glass, as they do in 

 proportion of the cube. This diflSculty not having been as 

 yet conquered, the large achromatic telescopes, the object- 



f lasses of whicli had moie than 48 Unes aperture, did not 

 ear a proportionate perfection to tiie smaller ones : and if still 



larger, 



