Life of John Dollond^ F. R. S. 4g 



recollected as always making a strong impression on thtf 

 minds of those with whom he conversed. His memory was 

 extraordinarily retentive, and, amidst the variety of his 

 reading, he could recollect and quote the most important 

 passages of every book which he had at any time perused. 



He^desiened his eldest son, Peter DoUond, for the same 

 business w^th himself; and for several years they carried on 

 their manufacture together in Spitalfields ; bat the employ- 

 ment neither suited the expectations nor dispositions of the 

 son, who, having received much information upon mathe- 

 matical and philosophical subjects from the instruction of 

 his father, and observing the great value which was set upoa 

 his father's knowledge "in the theory of optics by profes- 

 sional men, determined to apply that knowledge to the be- 

 nefit of himself and his family; and accordingly, under the 

 directions of his father, commenced optician. Success, 

 though under the most unfavourable circumstances, at- 

 tended eveiy effort; and in the year 1752 John Dollond, 

 embracinsr the opportunity of pursuing a profession conge- 

 nial witirhis mind, and' without neglecting the rules of 

 prudt'ncc towards his family, joined his sou, and in conse- 

 quence of his theoretical knowledge, soon became a profi- 

 cient in the practical parts of optics. 



His first attention was directed to improve the combina- 

 tion of the eye-gla?ses of refracting telescopes ; and having 

 succeeded m' his system of four eye-glasses, he proceeded 

 one step further, and produced telescopes furnished with 

 five eye-glasses, which considerably surpassed the former j 

 and of which he gave a particular account in a paper pre- 

 sented to the Royal Society, and which was read on tlie 

 istof March 1753, and printed in the Phil. Trans, vol. xjviii. 

 pace 103. 



Soon after this he made a vcr\' useful improveirent in 

 Mr. Savery's micrometer : for instead of caiploying two 

 entire object-glasses, as Mr. Savery and M. Bouguer had 

 done, he used only one glass cut into two equal parts^ 

 one of them sliding or movmg laterally by the other. This 

 was considered to be a great improveiiieut, as the micro- 

 meter could now be applied to the reflecting tclescoi-e with 

 much advantage-, andv.h.ch Mr. James Short immediately 

 did. An account of the same was given to the Royal So- 

 ciety, in a pi'per which was afterwards printed in the Phil* 

 Trans, vol. xlviii. page 17S*. 



♦ This kiad of micrometer was afterwards applied by Mr. P. Dollonl 

 to t; e achrunutic alcscopc. 



Vol. XVIII. No. 60. D ^^ 



