ON THE HISTORY OF OPTICS. 379 



of JHuygens in the mathematical theory of optics were no less considerable 

 than in the investigation of the nature of light ; his determinations of the 

 aberrations of lenses were the first refinement on the construction of tele- 

 scopes, but with respect to the theory of halos and parhelia he was less 

 successful than Mariotte had been some years before. 



In the year 1720, Dr. Bradley had the good fortune to discover both the 

 existence and the cause of the aberration of the fixed stars. He had for 

 some time observed an irregularity in the places of the stars, which he was 

 wholly unable to explain, and the idea of attributing it to a combination of 

 the effect of the earth's motion in its orbit, with the progressive motion of 

 light, occurred to him first as he happened to observe the apparent di- 

 rection of* the wind on board of a boat which was moving in a transverse 

 direction. He also determined with accuracy the magnitude of the at- 

 mospherical refraction,* which had been theoretically investigated by 

 Newton and by Taylor,t but never before practically ascertained with 

 sufficient precision. The formula, which Bradley appears to have deduced 

 from observation only, agrees precisely with an approximation which was 

 obtained by Simpson J from calculation ; but it cannot be considered as 

 rigidly accurate. 



The optics of Bouguer were first published in 1729, and an improved 

 edition appeared thirty years afterwards ; the merits of this author in the 

 examination of the properties of a variety of substances, with respect to 

 the transmission and reflection of light in different circumstances, and in 

 the comparison of lights of different kinds, require to be mentioned with 

 the highest commendation. Dr. Porterfield's|| investigations of the 

 functions of the eye tended greatly to illustrate the economy of this admi- 

 rable organ, and some valuable remarks of Dr. Jurin on the same subject 

 were soon after published in Dr. Smith's elaborate treatise on optics, which 

 contains all that had been done at that time with respect to the mathema- 

 tical part of the science. 



The invention of achromatic telescopes is with justice universally attri- 

 buted to our countryman Mr. Dollond,^[ but there is reason to believe that 

 he was not absolutely the first author of the improvement. Mr. Hall, a 

 gentleman of Worcestershire, is said to have discovered, about the year 

 1 729, Sir Isaac Newton's mistake, in supposing that the rays of different 

 colours must of necessity be equally separated by all surfaces which pro- 

 duce an equal mean refraction ; and by combining the different dispersive 

 properties of different kinds of glass, he constructed, in 1733, several com- 

 pound object glasses, which were calculated not only for avoiding all ap- 

 pearance of colour, but also for correcting the imperfect refractions of the 

 spherical surfaces of the separate lenses. He did not, however, make known 

 the particulars of his investigations, and his invention was soon wholly 

 forgotten. It was in consequence of a discussion ** with Euler, Klingen- 



* See Ph.Tr. 1787, p. 156. f Methodus Incrementorum, p. 108. 



J Mathematical Dissertations, 4to, 1743, p. 46. 



Hist, et Mem. 1726, H. 11 ; 1757, p. 1. Optique, 4to, Paris, 1760. 

 || On the Eye and Vision, 2 vols. Edin. 1759. 



^ Ph. Tr. 1. 735. Compare Newton's Optics, book i. part ii. Prop. 3, Exp. 8. 

 ** Hist, et Mem. 1756-7. 



