REFRACTION. 



Tycho Brahe's table of refraftion is as follows ; 



In this iUte the rcfraftion remained for many years. 

 Even Riccioli, in 1665, fiippofed it nothing at about 26^ of 

 altitude ; but he tliought the moon had only 79' of liori- 

 zoiital refradlion in fummer, the fun 30', and the Itars 

 30' 37". It was not till after the year 1672, that a tole- 

 rably near table of refraftion made its appearance, vvlien 

 the tldcr Caffini took the fubject into confideration. ( Mem. 

 de I'Acad. torn. v. p. 81.) What led to this was the 

 voyage of Richer to Cayenne in that year, upon the utility 

 of whiclv fome very excellent remarks were made by Caffini, 

 fliewing how far obfervations made in a fituation fo near 

 tlie equator tended to confirm or difprove certain theories 

 derived from obfervations made in Europe. Several very 

 ufeful deductions were drawn from a comparifon of thole 

 made both at Paris and Cayenne ; among others, the re- 

 fraftiou was fettled upon more accurate elements than herc- 

 tofore, and a new table computed, for the tirll time, of its 

 quantity, for all degrees, up to the zenith ; an abridgment 

 of which is given below : 



From the relation of his grandfon it appears, however, 

 that Caffini had at one time computed three tables of re- 

 fraftion for all altitudes ; one for winter, another for fum- 

 mer, and a third for fpring and autumn ; but fcveral 

 doubts having been fuggeiled to him refpefting this arrange- 

 ment, although in appearance conformable to nature, and 

 prmcipally the obfervations of Richer at Cayenne, where 

 the refraftion was found little different from that at Paris, 

 he changed his opinion ; and judging, that fince the great 

 difference of heat of the torrid zone from that of the 

 temperate zone, which we inhabit, does not caufe fenlible 

 differences in the refraftion ; therefore the greateft heat or 

 cold of our cHmate could not change it much ; and he then 

 fixed upon one table, which was that \ifed by the aflro- 

 nomers of the Royal Obfervatory of Paris up to the year 

 '745- 5 



It was always thought, before the tiir.t of Caffini, that 

 the refraction did not extend its influence higher tlian 45° 

 of altitude : and he is generally confidered as the firtl who 

 proved that it reached all the way to the zenith. He alfo 

 inppofed that near the equator the horizontal rtfraftion was 

 Icfs than in our climate by about one-tliird ; that this dif- 

 ference decreafcd as far up as 60", after which it waa the 

 fame nearly for both climates. 



From this difcovery it followed, as a natural confequence, 

 that the refraftion mult be greater near the pole than at 

 Paris : and this was fhortly afterwards proved to the 

 Academy by the publication of a work cxprefsiy on that 

 fubjeCt. (Refraftio folis inoccidui, &c. Holmise, 4to. 

 1695.) The king of Sweden, being, in 1694, at Toniea, 

 in Weil Bothnia, near the latitude of 65° 45', and obferving 

 that the fun did not fet there in the fummer folftice, fenl 

 the following year fome mathematicians to make more cer- 

 tam and exaft obfervations of this curious phenomenon. 

 Tiiey are contained in this book, andMefirs. Caffini and De 

 la Hire concluded from them, that in the latitude of 65° 45' 

 the horizontal refraftion mutt be 58', or nearly double of 

 that at Paris. 



According to an obfervation made by fome Dutchmen 

 who pafTed the winter of 1596 — 7, in Nova Zambia, in 

 latitude 76^ north, the fun, v/hich had entirely difappeared 

 the 14th of November, began to rife again the 24th of 

 .January, viz. fix days fooner than was expected, according 

 to aftronomical calculations. If fo, when the fun has been 

 two or three months under the horizon, as the Dutchmen 

 obferved in 1 597, the cold becomes dreadful, and perhaps 

 the refradtion increafes prodigioufly. M. le Monnier af- 

 fures us, that he found by the obfervations printed in 1599, 

 tliat on the 24th ai.d 27th of January 1597, there were more 

 than 4-1 degrees of refraction : that he could neither ex- 

 plain thefe obfervations, rejeft them as doubtful, nor fup- 

 pofe any error, as was done by molt of the other aftronomers, 

 Kepler, Caffini, Scote, and, laltly, M. le Gentil, in hi.s 

 Voy. dans les Mers des Indes, torn. i. p. 395. tom. ii. 

 p. 832, who maintained that there were errors in the ob- 

 fervations, and accordingly read a memoir on the fubjeft. 

 If it were not fo difiicult a taflc to winter in thefe high 

 latitudes, we might expedt fuch obfervations as would 

 remove all doubt on the fubjeft ; and, perhaps, bring others 

 to lij^rht of as great or greater importance. 



The refraftion of the north being fo confiderable, is very 

 ufeful to the inhabitants, who are deprived of the fun's 

 light during many months ; as it makes the fun rife much 

 earlier, and fet much later to them, than it otherwife would. 

 About the year 1725, Mr. Flamfteed, the Englifh aitro- 

 nomer royal, publilhed his table computed from his own 

 obfervations : and this was the one commonly ufed in 

 England for many years afterwards. 



Sir Ifaac Newton alfo conltrufted one from theory, 

 which was firft publifhed by Dr. Halley in the Philofophical 

 Tranfaftions, N° 368, for 1721. He made the horizontal 

 refraction 33' 45" ; whereas Mr. Flamiteed's was only 33' o". 

 But ahhough the refradion might be determined within 

 a few feconds at all altitudes by obfervation ; yet, the law 

 of its increafe from the zenith to the horizon was a fubjeft 

 that occupied the principal mathematicians and aftronomers 

 for more than a century. Newton having difcovered the 

 general principles of attra-ftion, found that the refraflion 

 was a confequence of this law of nature ; and that it arofe 

 from the attraftion of the atmofphere on the particles of 

 hght. On this principle the curve which a ray of light 

 defcribes might be determined ; fince it is fuccelljvely at- 



traded 



