260 The Achromatic Telescope. 



THE ACHEOMATIC TELESCOPE. 



In a previous paper upon this subject it was explained that 

 the defects of the old refracting telescope, with a single convex 

 lens as an object-glass, were inherent and irremediable. To 

 reduce their amount, therefore, as much as possible was the 

 object of the ancient opticians. Theory and practice concurred 

 in showing that they might be greatly diminished by con- 

 tracting the aperture of the object-glass ; that the intolerable 

 imperfection, for instance, of a three-inch lens, with a focus 

 of two feet, might be rendered almost insensible by " stopping 

 down," or limiting, its aperture to three-fourths of an inch ; 

 but, as this would involve so great a loss of light that the 

 instrument would be of little service in astronomical discovery, 

 the only way of making this palliative really available was to 

 reverse the process by preserving the aperture, but grinding 

 the lens to a longer focus. It was thus found that the three 

 inches would be brought to work well with a focal length ex- 

 tended to thirty feet; and in this way, with the increasing 

 demands of astronomers for greater light and power, these 

 instruments grew on and on till they reached such insufferably 

 cumbrous and unwieldy dimensions that, even though their 

 tubes had been discarded, they fairly broke down, as it were, 

 by their own length, and gave way to the more portable and 

 manageable reflector. 



. A still more convenient and generally-acceptable substitute 

 would, however, in all probability, have been found but for a 

 mistake, strange to say, of Sir Isaac Newton, the principal 

 one perhaps that was ever made by that wonderful man. 

 Although he had tried experiments on the refractive and dis- 

 persive powers of different media, the fact had escaped him — 

 as it seems, through an unfortunate selection of materials — that, 

 although the amount of dispersion always bears the same pro- 

 portion to that of refraction in the same substance, this does 

 not hold good in the case of different substances ; and accord- 

 ingly he concluded, and maintained his conclusion with a 

 pertinacity which must be regretted, that in every possible 

 case of refraction the separation into colour held the same 

 fixed proportion to it, and that consequently " the improve- 

 ment of telescopes of given lengths by refractions is desperate." 

 This singular error retarded the invention of the achromatic tele- 

 scope for about fifty-four years. It was not till 1 729 that Chester 

 More Hall, Esq., of More Hall, Essex, was led to the discovery 

 of the principle by the study of the wonderful structure of the 

 eye; and four years afterwards he completed an object-glass 

 of 2\ inches aperture, with a focus of about twenty inches, an 

 achievement which, had he thought fit to publish it, must have 



