ivith fluid object-glasses, 339 



scope, in proportion to their entire breadth, than those of the 

 primary spectrum produced by the refraction of a single me- 

 dium, on account of the more uniform density of the former. 

 A great part of the breadth of the primary spectrum is com- 

 posed of rays which are weak and inefficient, and incapable of 

 doing much injury to the distinctness. The secondary spec- 

 trum, on the contrary, consists of the whole primary spectrum 

 blended into two narrow but dense fringes, which do not fail 

 to produce their full bad effect upon the performance of the 

 telescope, as is very manifest when the aperture is at all con- 

 siderable. Upon these considerations probably, Mr Herschel 

 very judiciously recommends, that in the construction of achro- 

 matic telescopes a union should be formed, not of the extreme 

 red and violet, but rather of two of the intermediate colours, 

 such^ as the orange and green, which may be best done by 

 somewhat over-correcting (as it is termed) the primary colour, 

 by which means the blue and red will be united, and the vio- 

 let thrown out altogether, forming the least refracted ray. By 

 this means a greater concentration will be obtained of the more 

 effective rays of the spectrum at the expence of the diffusion 

 of those which are of less importance. I have heard my father 

 mention that he believed a very fine telescope belonging to the 

 late Mr Aubert owed its excellence to its being constructed by 

 accident in this way, as it exhibited a diffuse violet fringe round 

 the object on pushing in the eye-piece, instead of the usual 

 wine-coloured one. 



I have constructed several telescopes on my father's princi- 

 ple of twenty inches focal length, and two and three-quarter 

 inches aperture, in which the removal of both aberrations is 

 complete, so that the edges of the brightest object continue of 

 a perfectly pure white when the eye-glass is brought within or 

 without the focus, showing that the colour is corrected not only 

 for the lenses of the telescope, but for the humours of the eye 

 itself, thus rendering vision through the telescope more perfect 

 than the most perfect naked vision. This may perhaps be 

 thought extravagant, nevertheless it is strictly true, as the 

 chromatic aberration in the eye is far from being a small quan- 

 tity, and may easily be rendered visible. 



These instruments, notwithstanding their shortness, bear 



