AIDS OF THE NEWTONIAN PERIOD. 279 



aperture we suffer from loss of light. What remains 

 then is to increase the focal length. This was done 

 to an extraordinary extent, in telescopes constructed 

 in the beginning of the last century. Huyghens, in 

 his first attempts, made them 22 feet long 4 ; after- 

 wards, Campani, by order of Louis the Fourteenth, 

 made them of 86, 100, and 136 feet. Huyghens, 

 by new exertions, made a telescope 210 feet long. 

 Auzout and Hartsoecker are said to have gone 

 much further, and to have succeeded in making an 

 object-glass of 600 feet focus. But even such tele- 

 scopes as those of Campani are almost unmanage- 

 able : in that of Huyghens, the object-glass was 

 placed on a pole, and the observer was placed at 

 the focus with an eye-glass. 



The most serious objection to the increase of 

 the aperture of object-glasses, was the coloration of 

 the image produced, in consequence of the unequal 

 refrangibility of differently-coloured rays. Newton, 

 who discovered the principle of this defect in lenses, 

 had maintained that the evil was irremediable, and 

 that a compound lens could no more refract without 

 producing colour, than a single lens could. Euler 

 and Klingenstierna doubted the exactness of New- 

 ton's proposition; and, in 1755, Dollond disproved 

 it by experiment. This discovery pointed out a 

 method of making object-glasses which should give 

 no colour ; which should be achromatic. For this 

 purpose Dollond fabricated various kinds of glass 

 4 Bailly, ii. 253. 



