478 LECrUKE XL. 



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 direction 

 of the wind on board of a boat which was moving in a transverse direction. 

 He also determined with accuracy the magnitude of the atmospherical refrac- 

 tion, which had been theoretically investigated by Newton and by Taylor, 

 but never before practically ascertained with sufficient precision. The for- 

 mula, which Bradley appears to have deduced from observation only, agrees 

 precisely with an approximation which was obtained by Simpson from calcu- 

 lation; 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 exmai- 

 nation of the properties of a variety of substances, with respect to the trans- 

 mission and reflection of light in different circumstances, and in the comparison 

 of lights of different kinds, require to be mentioned with the highest commen- 

 dation. Dr. Porterfield's investigations of the functions of the eye tended 

 greatly to illustrate the economy of this admirable 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 mathematical part of the science. 



The invention of achromatic telescopes is with justice universally attri- 

 buted to our countryman Mr. DoUond, but there is reason to believe that he 

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

 tleman of Worcestershire, is said to have discovered, about the year 1729, Sir 

 Isaac Newton's mistake, in supposing that the rays of different colours must 

 of necessity be equally separated by all surfaces which produce an equal 

 mean refraction ; and by combining the different dispersive properties of 

 different kinds of glass, he constructed, in 1733, several compound object 

 glasses, which were calculated not only for avoiding all appearance 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 



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