On Refraction. 341 



sketch, the various endeavours of these illustrious men, is 

 the humble attempt or this short essay ; which, it is hoped, 

 will have the desired effect of stimulating others, who pos- 

 sess the means, to the consideration of the subject, that 

 we may shortly be enabled to discover its quantity with the 

 greatest accuracy, at all altitudes, and under all changes of 

 the atmosphere. 



There appears to be but little doubt that the astronomic 

 refraction was known to the ancients, since it is expressly- 

 mentioned by Ptolomy, although not made use of in his 

 calculations *. He says, near the end of the eighth book of 

 the Almagest, that in the rising and setting of the heavenly 

 bodies there are changes which depend upon the atmosphere: 

 and he mentioned it more at length in a work on optics 

 which unfortunately has not been handed down to usf . 



Alhazen, an Arabian writer, who is generally supposed 

 to have lived about the vear 1100, and to have taken the 

 greater part of his optics from the works of Ptolomy, speaks 

 also decidedly of it, and shows the manner of convincing 

 ourselves o its existence by experiment J. 



" Take/' he says, "an armillary sphere which turns round 

 its poles, and measure the distance of a star from the pole of 

 the world when it passes near the zenith in the meridian, 

 and when it is rising or setting near the horizon, and you 

 will find the distance from the pole less in the latter case." 

 He then demonstrates that this must arise in consequence 

 of the refraction, but he does not state its quantity. 



In the collection of observations made by Bernard Wal- 

 ter, published by Willebrord Snell, in the year 1618, it is 

 stated, the observations were so exact that they pointed out 

 to Walter the quantity by which the altitudes of the stars 

 and planets were increased on account of the refraction. 



TychoBrahe§, however, appears to have been the first 

 who asserted, with any degree of accuracy, that the refrac- 

 tion elevates the heavenly bodies rather more than half a 

 degree when in the horizon. But either his instruments or 

 his observations were not sufficiently correct to determine 

 it with certainty for all degrees from the zenith to the ho- 

 rizon : and accordingly where these failed the rest was sup- 

 plied bv conjecture. He believed that the sun's refraction 

 was 34' in the horizon, and that it became insensible at 45° 

 of altitude. For the stars, however, he assumed an en- 



* La Lande's Astronomy, 2163, 3d edit. Encyciop. Yverd. art. Rrfrnctivn. 

 Encycl. Mcth. do. a 



+ Lh Lande's Astronomy, as above. Smith's Optics, p. 58. Remarks. 

 Friestley's Hist. Opt. 4to, p. 18. 



\ Encyciop. Yverd. art. Refraction. § Progymn. p. 15. 



Y 3 tirely 



