UEFRACTION. 



ileiilities ; yel uiittuoub and lulpluucoub bodks are lound to 

 refled more Itroiigly than accoidiug to llieir mere denfi- 

 ties ; for as the rays aft more ilrongly on thofc Ixidies to 

 kindle them than on others, ib do they again, by their 

 mutual attraflion, aft more (Irongly on the rays to refraft 

 them. 



Laftly ; Bccaufe not only tliofe rays tranfmittcd througli 

 glafs are found to be refrafted, but alfo thofe palling in the 

 air, or in a vacuum near its extremities, or even near the 

 extremes of many opaque bodies, e. gr. the edge of a knife, 

 undergo a fimilar infleftion, from the attraction of the body. 

 See Light, and alfo Inflection. 



The manner in which refraftion is performed by mere at- 

 traftion, without contadt, may be thus accounted for : iup- 

 pofe H 1 {Plate XVIII. Optics, fig. ii.) tlie boundary t>f 

 two mediums, N and O ; the firll the rarer, c. gr. air ; the 

 fecond the denfer, c. gr. glafs ; the attradtions of the me- 

 diums here will be as their denfities. Suppofe /> S to be the 

 dillance to which tlic altradUng force of the denfer mediums 

 exerts itfelf within the rarer. Let now a ray of light A a 

 fall obliquel)- on the fu-rface which feparales the mediums, 

 or rather on the furface/> S, where the adlion of the fecond 

 and more refilling medium commences. All attradtion being 

 performed in lines perpendicular to the atlradtive body, as 

 the ray arrives at a, it will begin to be turned out of its rec- 

 tilinear courfe by a fuperior force, with which it is attradted 

 by the medium O, more than by the medium N, i. e. by a 

 force with which it is driven towards it in a diredtion perpen- 

 dicular to its furface : hence the ray is bent out of its right 

 line in every point of its paflage between p S and R T, 

 within which diilance the attradlion adls. Between thofe 

 lines, therefore, it defcribes a curve ah6 ; but beyond R T, 

 being out of the fphere of attradiion of the medium N, it 

 will proceed uniformly in a right hue, according to the di- 

 redtion of the curve in the point b. 



Again, fuppofe N the denfer and more refilling medium, 

 O the rarer, and H I the boundary, as before ; and let R T 

 be the dillance to which the denfer medium exerts its attrac- 

 tive force within the rarer : even when the ray has palled the 

 point B, it will be within the fphere of fuperior attradlion 

 of the denfer medium ; but that attradtion adling in lines 

 perpendicular to its furface, the ray will be continually 

 drawn from its ftraight courfe B M perpendicularly towards 

 H I : thus, having two forces or diredtions, it will have a 

 compound motion, by which, inilead of B M, it will de- 

 fcribe B m, which B m will in ftridlnefs be a curve. 



Laftly, after it has arrived in m, being out of the in- 

 fluence of the medium N, it will perfift uniformly in a right 

 line, in the diredtion in which the extreme of the curve 

 leaves it. 



Thus we fee how refradlion is performed, both towards 

 the perpendicular, 3.nd from it. 



But note, the attradtion of die denfer medium, e.gr. N, 

 is continually diminifhing, as the ray proceeds from B, to- 

 wards the Hmit of attradtion R T ; becaufe fewer and fewer 

 parts ftill come to adt ; at 1 H, e. gr. all the parts between 

 that and p S attradt ; but at R T, none but thofe in the line 

 H I. Note, alfo, that the diftance between /iS and RT 

 being fmall, when we confider refradtions, no notice is taken 

 of the curve part of the ray, but we confider it as confifting 

 of two ftraight lines, C B, A B, or m B, A B. 



Sir Ifaac Newton, not content with afcribing the reflec- 

 tion, refradtion, and infledtion of light to powers of attrac- 

 tion and repulfion, extending beyond the furfaces of bodies 

 and producing effedt in the manner above explained, propofes 

 a conjedture concerning the phyfical caufe of this attrac- 

 tion and repulfion ; but his hypothefis is no lefs liable to 

 difficulties and objeftions than the hypothefis of the me- 



chanical production of the motion of light without attrac- 

 tion or repulfion. Does not the refradtion of light, he fays, 

 arife from the different deufity of an etiierial medium in dif- 

 ferent places, the light always receding from the denfer 

 parts of the medium ? And is not the denfity of it greater 

 in free and open fpaccs void of air, and other grofs bodies, 

 than within the pores of water, glafs, cryftal, gems, and 

 other compadt bodies ? For when light pafles through gUfs 

 or cryllal, and falling very obliquely upon the farther fur- 

 face, is all refledted, the total refledtion ought to ariCe rather 

 from the denfity and vigour of the medium without and be- 

 yond the glafs, than from the rarity and.weaknefs of it. — 

 Does not the etherial medium, in paffing out of water, glafs, 

 cryllal, and other compadt and denfe bodies, into empty 

 (paces, grow denfer and denfer by degrees, .-ind by that 

 means rcfradt the rays of light, not in a point, but by bend- 

 ing them gradually in curve lines ? And does not the 

 gradual condenfation of this medium extend to fome dif- 

 tance from the bodies, and thereby caufe the infledtions of 

 the rays of light, which pafs by the edges of the denfe 

 bodies, at fome diftance from the bodies ? 



Refraction, in Dioptrics, is the infledtion or bending of 

 the rays of light, in paffing the furfaces of glafl'es, lenfes, 

 and other tranlparent bodies of different denfities. 



Thus a ray, as A B (Plate XVf II. Optics, Jig. 1 1.) falhng 

 obliquely from the radiant A, upon a point B, in a dia- 

 phanous furface, H I, rarer or denfer than the medium 

 along which it was propagated from the radiant ; has its di- 

 redtion there altered by the adtion of the new medium ; and 

 inttead of proceeding to M, it deviates, e.gr. to C. 



This deviation is called the refra3ion of the ray / B C the 

 refrnSkd ray,- or line of refratlion ; and B the point of re- 

 fraSion. 



The line A B is called the line of incidence, or ray of in- 

 cidence ; and, in refpedt of it, B is alfo called the point of 

 incidence. 



The plane in which both the incident and refradted rays 

 are found, is called the plane of refraction ; a right line B E, 

 drawn in the refradting medium perpendicular to the refradl- 

 ing furface, in the point of refradtion B, is called the axis of 

 ref radian : and a right line D B, drawn perpendicular to the 

 rafradting furface, in the point of incidence B, along the 

 medium through which the ray fell, is called the axis of 

 incidence. 



The angle A B I, included between the incident ray and 

 the refradting furface, is called the angle of incidence; and 

 the angle A B D, included between the incident ray and the 

 axis of incidence, is called the angle of inclination. The 

 angle M B C, which the refradted ray makes with the inci- 

 dent, is called the angle of refaction : and the angle C B E, 

 which the refradted ray makes with the axis of refradtion, is 

 called the refraSed angle. 



Refraction, general laws of l. yi ray of light in its 

 parage out of a rarer into a dtnfer medium, e. gr. out of air 

 into glafs, is refraSed towards the perpendicular, i.e. towards 

 the axis of refra8ion. 



Hence, the refradted angle is lefs than the angle of in- 

 clination ; and the angle of refradlion lefs than that of in- 

 cidence ; as they would be equal were the ray to proceed 

 ftraight from A to M. Hence, alfo, a ray perpendicular 

 to the refradting furface, will pafs llirougii without being 

 refradted, as it cannot be refradted to the perpendicular. 

 The phyfical caufe of which is, that the attradtion of the 

 denfer medium, wliich, in an incidence oblique to its fur- 

 face adts perpendicularly to that furface, draws the ray 

 out of its courfe : this attradtion, we fay, ia a perpendi- 

 cular incidence, adts in the diredtion of the ray. 



2. The ratio of the fine of the angle of inclination, to the 

 lo fne 



