454 The Achromatic Telescope. 



any irregularity in this respect having the effect of refracting the 

 rays which encounter it more or less than their neighbours, 

 and thus seriously impairing their accurate conveyance ; good 

 workmanship, too, is highly important — not merely brilliancy 

 of polish, which, though very desirable, is not essential, but 

 still more accuracy of form, any deviation from the correct 

 curve being fatal to excellence. But, supposing all these 

 combined, a single convex lens will not, after all, produce an 

 accurate picture, or, optically speaking, form a good object- 

 glass : and the cause of this imperfection may be well illustrated 

 by means of a common reading or burning-glass. If we hold 

 it in the sun and examine carefully, by receiving them upon a 

 card or piece of paper, the appearance of the rays as they pro- 

 ceed towards and beyond the focus, we shall find, 1, that the 

 circumference of the bright circle formed upon the paper is 

 more luminous and sharply defined before reaching the focus 

 than afterwards ; — and, 2, that in the former case the circle is 

 bordered by a narrow fringe of reddish orange, which beyond 

 the focus is changed to a bright blue. Either of these pecu- 

 liarities injures the distinctness of the picture, and each requires 

 separate consideration. 



As to the first defect, the condensation of light towards the 

 edge of the rays converging to focus, it arises from the fact 

 that it is not possible for a spherical surface, or any portion of 

 one, to collect the rays passing through it in one point. The 

 cause of this cannot be explained without a considerable know- 

 ledge of the mathematical law of refraction, upon which we 

 must not here enter; it is sufficient to state that it requires 

 that the rays refracted at a spherical surface should converge to 

 foci nearer and nearer to that surface, in proportion as the sur- 

 face is more highly inclined to the direction of the incident or 

 emergent ray. Hence, the focus of the margin of a convex 

 lens is perceptibly shorter than that of its centre — a fact which 

 may be experimentally shown by so covering a reading or 

 burning-glass with paper, that a narroAV ring shall be left ex- 

 posed all round the edge, and a small opening in the centre : if 

 the lens is then held in the sunshine, and a card moved back- 

 wards and forwards through the focus (care being taken to 

 keep both lens and card at right angles to the direction of the 

 solar rays), it will be found that the ring formed by the 

 marginal rays will convorgo to a point short of that reached by 

 those from the centre, and that, being crossed by these latter 

 rays, it will not be sharply defined ; while the image from the 

 central opening, formed further on and more distinct, will be 

 surrounded willi Uio re-opening luminous ring.* This defect, 



* This experiment may be tried by candle-light, as a strong sunshine would 

 be found very dazzling. 



