PHOTOGRAPHING THE STARS. 307 



are brought by a single lens is different from that in which 

 a beam of red would be concentrated. The blue focus is 

 nearer to the lens than the red focus. There is here a 

 radical difference between the action of the lens upon 

 light and the action of the mirror. In the latter case, 

 every hue, of whatever colour, if in the visible part, or 

 indeed whether the rays belong to the visible portion of 

 the spectrum or not, is brought to coincidence at the 

 same point. The glass lens, however, has a different 

 focus for every different quality of light which can fall 

 upon it. Hence, when a beam from the sun or from a 

 star, or indeed from almost any celestial source, falls upon 

 a lens of glass, the composite nature of the light gives 

 rise to the difficulty that the reds, the yellows, and the 

 blues are all brought to different foci. It is therefore 

 impossible for this reason to obtain a distinct and 

 definite image of any celestial object with a single 

 glass lens; for if the lens be focussed truly for some 

 of the rays it is necessarily out of focus for others. 

 This difficulty is well known, and was for a long time 

 regarded as presenting an insuperable difficulty in the 

 way of constructing efficient refracting telescopes with 

 glass lenses. Indeed, it was the perception of this diffi- 

 culty that led Newton to turn his attention to the con- 

 struction of reflectors, the best known form of which still 

 bears his name. By the admirable discovery that what a 

 single lens could not do a pair of glasses could certainly 

 accomplish, the refracting telescope was made the valuable 

 instrument we now find. The achromatic objective is 

 formed of a lens of crown glass and a lens of flint glass. 

 A beam of composite light, on passing through a powerful 



