Life of John Dollond, F.R.S. ai 



main satisfied with argviing in this manner from an experi- 

 ment made by another, but determined to try it himself: 

 and, accordingly, in the year 1757, began the examination; 

 and, to use his own words, with '* a resolute perseverance," 

 continued during that year, and a ^reat part of the next, to 

 bestow his whole mind on the subject, until in the month 

 of June 1758 he found, after a complete course of experi- 

 ments, the result to be very different from that which he 

 expected, and from that which Sir Isaac Newton had re- 

 lated. He discovered " the difference in the dispersion of 

 the colours of light, when the mean rays are equally refracted 

 ly dijferent mediums." The discovery was complete, and 

 he immediately drew from it this practical conclusion, 

 *' That the object-glasses of refracting telescopes were ca- 

 pable of being made without being aflected by the different 

 refrangibility of the rays of light." His account of this 

 experiment, and of others connected with it, was given to 

 the Royal Society, and printed in their Transactions, vol. 1. 

 page 743. and he was presented in the same year, by that 

 learned body, with Sir Godfrey Copley's medal, as a re- 

 ward of his merit, and a memorial d the discovery, though 

 not at that time a member of the society. 



This discovery- no way affected the points in dispute 

 between Euler and Dollond, respecting the doctrine ad- 

 vanced by Sir Isaac Newton. A new principle was in a 

 manner found out, which had no part in their former rea- 

 sonings, and it was reserved for the accuracy of Dollond to 

 have the honour of making a discovery which had eluded 

 the obsei-vation of the immortal Newton *. 



This new principle being now established, he was soorx 

 able to construct object-glasses, in which the different re- 

 frangibility of the rays of light was corrected, and the 

 name of achromatic given to them by the late Dr. Bevis, 

 on account of their being free from tlic prismatic colours. 

 Dr. Hutton, in his Mathematical Dictionary, has said that 

 this name was given to them by M. de Lalandej but that 

 is a mistake. 



As usually happens on such occasions, no sooner was 

 the achromatic telescope made public, than tlie rivalship of 



* The cav.se of this difference of the results of the 8th expeiiment of 

 the ind part of the first book of Newton's Optics, as rcLued by himself. 

 Bin) as it was found when tried by Dollond in the years 1757 unci 1758, 

 is fully and ingeniously accounted for by Mr. Pitjr Dollond in a papei: 

 read at tiie Royal Society on the zisc of May, 17S9, and afterwards 

 puhlisucd for J. Johnson' in St. Paul's Chuicii Yard; also in Hutt<j.rf« 

 fejctionary — Anisic, Chromatic, 



D 2 foreigners, 



