456 The Achromatic Telescope. 



ing upon a reading- glass a circular piece of paper of a somewhat 

 smaller size as in the previous experiment, only omitting the 

 central hole, and holding it up to the sun ; the interior of the 

 cone being thus stopped out, each side of the remaining ring 

 •will be found bordered with colour, and the red and blue fringes 

 will change places in passing through the focus.* This separa- 

 tion into colour is technically termed " chromatic aberration," 

 or more frequently " dispersion." 



From the combination of these two sources of error, the 

 focus of a simple refracting telescope, instead of being, as it 

 ought to be, a single point uniting the whole body of con- 

 verging rays, is a circle of appreciable magnitude, being the 

 smallest common section of a number of cones meeting and 

 crossing each other in different points, its size being deter- 

 mined by the distance between the extreme foci on either side 

 of it : and consequently the image of every point in the object 

 viewed will be a small, discoloured circle ; and every outline in 

 nature, which may be considered as made up of contiguous 

 points, will be misty and ill- defined in the focal image, and 

 exhibiting its imperfection still more in proportion as it is 

 magnified by the eye-lens, will render telescopic performance 

 very unsatisfactory. We must now consider what remedy may 

 be applied to each of the defects whose united influence is so 

 prejudicial. 



1. The spherical error can be perfectly removed in theory, 

 as was pointed out by Descartes, by making the exterior sur- 

 face of the lens a portion of an ellipsoid or hyperboloid, instead 

 of a sphere. But the extreme difficulty of working glass to 

 any other than a spherical or flat surface rendered, this dis- 

 covery of no practical use : the amount of the aberration indeed, 

 varying with the curves of the surfaces, may be much reduced 

 by so combining them as to produce a minimum effect ; but it 

 never can be altogether annihilated. 



2. Dispersion, which is the far more serious evil of the two, 

 as introducing a much greater angle of deviation, is perfectly 

 irremediable in its own nature, being an inseparable portion of 

 the refraction by which the focal image is formed. Some 

 transparent media possess a greater, others a less dispersive 

 power; or, in other words, the interval between the red and 

 blue foci differs somewhat according to the material of the lens : 

 but the substancos which occasion less proportional dispersion 

 than glass are too expensive, or too unmanageable, to be employed 

 with advantage, the difference, after all, being of no great amount. 



* This experiment may be varied in a pleasing manner by covering the wholo 

 lens with a piece of paper, pierced with a great number of small holes, chiefly, 

 however, towards the edges, as near the centre the discolouration will scarcely bo 

 perceptible. 



