REFRACTION. 



ihe lurfacc of tlie glafs, be fhewii to be refr.ifted fo as to 

 meet the if(l about the point G. See Focus and Lf.ns. 



Hence the great property of convex glafles ; viz. that they 

 Lulletl parallel rnys, or miiin thtm converge into a point. 



Refkaction in a concave lens. Parallel rays A B, CD, 

 and EF (^g. 23.) falling on a concave lens G B H I M K, 

 the ray A B, falling peqieudicular on the glafs at B, will 

 pafs unrefradted to M ; where being ftiU perpendicular, it 

 will pafs into the air without refraftion, to I^. But the 

 ray C D, falling obliquely on the furface of the glafs, will be 

 refrafted towards the perpendicular N D O, and proceed to 

 Q ; and the ray D Q, again falling obliquely out of the 

 glafs upon the furface of air, will be refrafted from the 

 perpendicular R O S, and proceed to V. After the fame 

 maaner might the ray E F be fhewn to be refraftcd to Y, 

 and thence to Z. 



Hence the great property of concave glaffes ; vif^. that they 

 difperfe parallel rays, or make them diverge. See Lens and 



MiKROR. 



Refraction in a plane vhifi. If parallel rays E F, G H, 

 1 L {Jig. 24.) fall obliquely on a plane glafs A B C D, the 

 obliquity being the fame in all, by reafon of their parallelifm, 

 they will be all equally refrafted towards the perpendicular ; 

 and accordingly, being Hill parallel at M, O, and Q, they 

 ■will pafs out into the air equally refracted again from the 

 perpendicular, and ftill parallel. 



Thus will the rays E F, G H, and I L, at their entering 

 the glafs, be inflefted towards the right ; and in their going 

 out as much inflefted to the left ; fo that the firft; refradlion 

 is here undone by the fecond ; though not fo as that the ob- 

 jeft is feen in its trueplace. For the ray B Q, being produced 

 back again, will net coincide with the ray L I, but will fall 

 to the right of it ; and this the more as the glafs is thicker ; 

 however, as to matter of colour, the fecond refraftion does 

 really undo the firft. See Colour. 



Refraction of Heat. See Heat, Light, and Rays of 

 Heat. 



Refraction in Iceland or IJland cryjlal. See Iceland 

 Crystal. Dr. Young, who maintains that radiant light 

 confifts in undulations of the luminiferous ether, takes oc- 

 cafion to make fome remarks on fir Ifaac Newton's theory 

 of the peculiar refraction in Iceland cryftal. Newton, he 

 fays, has advanced the fingular refraftion of the Iceland 

 cryftal, as an argument that the particles of light mull be 

 projefted corpufcles ; fince he thinks it probable that the 

 different fides of thefe particles are differently attrafted by 

 the cryftal, and fince Huygens has confefied his inability to 

 account, in a fatisfaftory manner, for all the phenomena. 

 But contrarily to what might have been expefted from New- 

 ton's ufual accuracy and candour, he has laid down a new 

 law for the refraftion, without giving a reafon for rejefting 

 that of Huygens, which Mr. Haiiy has found to be more 

 accurate than Newton's ; and, without attempting to deduce 

 from his own fyftem any explanation of the more univerfal 

 and ftriking eifefts of doubling fpars, he has omitted to ob- 

 ferve, that Huygens's moft elegant and ingenious theory 'per- 

 feftly accords with thefe general effects, in all particulars, 

 and of courfe derives from them additional pretenfions to 

 truth ; this he omits, in order to point out a difficulty for 

 which only a verbal folution can be found in his own theory, 

 and which will probably long remain unexplained by any 

 other. 



Dr. Wollafton, in his paper on the oblique refraftion 

 •of Iceland cryftal, confirms the experiments of Huygens on 

 this fubftance, with additional evidence, deduced from the 

 -fuperiority of his mode of examining the powers of re- 

 fraction. He obferves that Dr. Young has already apphed 



the Huygenian theory v/ith confiderable fucccfs to the ex- 

 planation of feveral other optical phenomena, and that it 

 appears to be ftrongiy Supported by lucii a coincidence of 

 "■he calculations deduced from it, with the refnlts of thefe ex- 

 periments, as could hav' Icarcely liappened to a falfe theory. 



In ordinary cafes, the incipient undulations are of a fphe- 

 rical form ; but in the Iceland cryftal light appeared to Huy- 

 gens to proceed as if the undulations were portions of an ob- 

 late fpheroid, of which the axis is parallel to the ftiort dia- 

 gonal of an equilateral piece of the cryftal, and its centre 

 the point of incidence of its ray. From this fpheroidat 

 form of the undulations, he deduces the obhquity of refrac- 

 tion ; and lays down a law obfervable in all refraftions, at 

 any furface of the fpar, whether natural or artificial, which 

 bears the clofeft analogy to that which obtaiss, univerfaUy, 

 at other refraftory furfaces ; for as, in other cafes, the ratio 

 is given between the fine of incidence and fine of refraftion, 

 (or ordinate of thej^/;f/-/ra/ undulation propagated,) fo lu 

 the Iceland cryftal, the ratio between the fine of incidence 

 and ordinate of refraftion ^in any one feftion of the fphe- 

 r&«/(tW undulation) in a given ratio, but diflerent in different 

 planes. 



Dr. Wollafton obferves, that though we do not fully 

 undcrftand the exiftence of a double refraftion, and are 

 utterly at a lofs to account for the phenomena occumng 

 upon a fecond refraftion, by another piece of the fpar, yet 

 that the obhque refraftion, when confidered alone, is nearly 

 as well explained as any other optical phenomenon. Phil. 

 Tranf. for 180 1 and for 1802. 



Refraction, Particular laws of, in different kinds of 

 lenfes ; fee Lens. 



Refraction, Atmofpherical, is generally confidered under 

 two diftinft heads ; viz. aftronoinical refraftion, which is 

 that relating to the refraftion of the moon, ftars, and other 

 celcftial bodies ; and tcrreftrial refraftion, or that which 

 takes place in terreftrial obfervations. 



It appears, from the article Refraction, that a ray of 

 light is refrafted in paffmg obhquely out of one medium into 

 another of different denfity ; and as the atmofphere may be 

 confidered as corapofed of an infinitude of ftrata, whofe 

 denfity increafe as they are pofited nearer the earth, the lu- 

 minous rays which pafs through it are afted on as if they 

 pafled fucceffively through media of increafing denfity, and 

 are therefore inflefted more and more towards the earth, as 

 the denfity augments, that is, as they approach the eye of 

 the obferver. In confequence of this it is, that rays from 

 objefts, whether celeftial or terreftrial, proceed in curves 

 which are concave towards the earth ; and fince the mind al- 

 ways refers the place of objefts to the direftion in which 

 the rays reach the eye, that is, to the direftion of the tan- 

 gent to the curve at that point, Jt follows, that the apparent 

 or obferved elevation of objefts is always greater than the 

 true one, at leaft with the exception of fome few remark- 

 able anomalies which fometimes, in terreftrial obfervations, 

 produce a contrary effeft, and of which we have ftated a 

 few particular inftances under the article Mirage, and on 

 which fubjeft fome other curious fafts are recorded in a me- 

 moir of profedor Vince's, in the Phil. Tranf. ; in Nicholfon's 

 Philofophical Journal, 4to. ; and more particularly by 

 M. Monge, in vol. i. of the " Decade Egyptienne." Re- 

 ferring the reader therefore to thofe works, to which we 

 ought alfo to add the memoir by M. Biot, in vol. x. of the 

 National Inftitute, for an account and explanation of thofe 

 phenomena, we ftiall confine ourprefent obfervations to thofe 

 cafes only, in which an uniform law is fuppofed to have place, 

 and which, as we have fecn, has a conftant tendency to aug- 

 ment the obferved altitude of both tcrreitrial and cileftiai 



objefts. 



