PLEASURES OE THE TELESCOPE. 217 



serve to reveal what could, not be seen without it, the image itself 

 must be as nearly perfect as possible ; this requires that every ray 

 of light that forms the image shall be brought to a point in the 

 image precisely corresponding to that from which it emanates in 

 the real object. In reflectors this is effected by giving a para- 

 bolic form to the concave surface of the mirror. In refractors 

 there is a twofold difficulty to be overcome. In the first place, a 

 lens with spherical surfaces does not bend all the rays that pass 

 through it to a focus at precisely the same distance. The rays 

 that pass near the outer edge of the lens have a shorter focus 

 than that of the rays which pass near the center of the lens ; this 

 is called spherical aberration. A similar phenomenon occurs with 

 a concave mirror whose surface is spherical. In that case, as we 

 have seen, the difficulty is overcome by giving the mirror a para- 

 bolic instead of a spherical form. In an analogous way the 

 spherical aberration of a lens can be corrected by altering its 

 curves, but the second difficulty that arises with a lens is not so 

 easily disposed of : this is what is called chromatic aberration. It 

 is due to the fact that the rays belonging to different parts of 

 the spectrum have different degrees of refrangibility, or, in other 

 words, that they come to a focus at different distances from the 

 lens ; and this is independent of the form of the lens. The blue 

 rays come to a focus first, then the yellow, and finally the red. It 

 results from this scattering of the spectral rays along the axis of 

 the lens that there is no single and exact focus where all meet, and 

 that the image of a star, for instance, formed by an ordinary lens, 

 even if the spherical aberration has been corrected, appears 

 blurred and discolored. There is no such difficulty with a mirror, 

 because there is in that case no refraction of the light, and con- 

 sequently no sjDlitting up of the elements of the spectrum. 



In order to get around the obstacle formed by chromatic aber- 

 ration it is necessary to make the object glass of a refractor con- 

 sist of two lenses, each composed of a different kind of glass. 

 One of the most interesting facts in the history of the telescope is 

 that Sir Isaac Newton could see no hope that chromatic aberra- 

 tion would be overcome, and accordingly turned his attention to 

 the improvement of the reflecting telescope and devised a form of 

 that instrument which still goes under his name. And even 

 after Chester More Hall in 1729, and John Dollond in 1757, had 

 shown that chromatic aberration could be nearly eliminated by 

 the combination of a flint-glass lens with one of crown glass, 

 William Herschel, who began his observations in 1774, devoted 

 his skill entirely to the making of reflectors, seeing no prospect of 

 much advance in the power of refractors. 



A refracting telescope which has been freed from the effects of 

 chromatic aberration is called achromatic. The principle upon 



VOL. XLV. 17 



