REFRACTION. 



Voflius indeed, and SncUius, imagined they had obfcrved 

 a perpendicular ray of light undergo a refraftion ; a perpen- 

 dicular oh]o&. appearing in the water nearer than in reality 

 it was ; but this was to attribute that to a refraftion of the 

 perpendicular rays, which was owing to the divergency of the 

 ttblique rays after refraftion, from a nearer point. 



Yet there is a manifett refradtion even of perpendicular 

 rays found in I/land Cryhtal, which fee. 



Rohault adds, that though an oblique incidence be necef- 

 fary in all other mediums we know of, yet the oblique muft 

 not exceed a certain degree ; if it do, the body will not pene- 

 trate the medium, but will be refleaed inaead of being 

 refrafted. 



Thus cannon-balls, in fea-engagements, ialhng very 

 obliquely on the furface of the water, are obf«-ved to mount 

 aloft again, and frequently to fweep the men from ofT the 

 oppofite decks ; and the hke happens to the little ilones 

 with which children make their ducks and drakes. 



The ancients confounded refraftion with refleftion, and 

 it was fir Ifaac Newton who lirft taught us the jull dilFerence 

 between them. He (hews likewife, that there is a good 

 deal of analogy between them, and particularly in the cafe 

 of light- 



The laws of refraftion of the rays of light in mediums dif- 

 ferently terminated, /. e. whofe furfaces are plain, concave, 

 convex, &c. make the fubjeft of dioptrics ; which fee. 



By refraftion it is, that convex glafles, or lenfes, col- 

 left the rays, magnify objefts, burn, &c. and hence the 

 foundation of microfcopes, telefcopes, &c. 



By refraftion it is, that all remote objefts are fcen out of 

 their real places ; particularly, that the heavenly bodies are 

 apparently higher than they are in reality, &c. 



The refraftion of the air has many times fo uncertain an in- 

 fluence on the places of celeftial objefts, very remote from the 

 zenith, that wherever refraftion is concerned, the conclufions 

 deduced from obfervations that are much affefted by it, will 

 always remain doubtful, and too precarious in many cafes to 

 be relied upon. See Dr. Bradley, in Phil. Tranf. N°485. 

 See Atmofpherical REFRACTlOf. 



Refraction of Light, in Optics, is an infleftion or devia- 

 tion of the rays from their reftilinear courfe upon falling 

 obliquely out of one medium into another, of a different 

 denfity. 



The term refraftion is derived from the diftortion which it 

 occafions in the appearance of an objeft viewed in part only 

 by refrafted light ; thus, an oar, partially immerfed in water, 

 appears to be bent, on account of the refraftion of the light, 

 by which its lower part is feen, in its pafl'age out of the 

 water into the air. 



Although no fenfible light can penetrate more than 

 700 feet deep into the fea, and a length of feven feet of 

 water has been found to intercept one-half of the light 

 which enters into it ; yet in tranfparent fubllances (no me- 

 dium being, Itriftly fpeaking, abfolutely tranfparent) the 

 greater part of the light penetrates to all diftances with httle 

 interruption, and all rays, of the fame kind, thus tranf- 

 mitted by the fame furface, form with the perpendicular an 

 angle of refraftion, which is ultimately in a certain conltant 

 proportion to the angle of incidence ; that is, for inflance, 

 one-half, three-fourths, or two-thirds, according to the na- 

 ture of the furface. Thus, if the refraftive properties of 

 the fubftance were fuch, that an incident ray, making an 

 angle of one degree with the perpendicular, would be fo 

 refrafted as to make an angle of only half a degree with 

 the fame line, another ray, incident at an angle of two de- 

 grees, w-ould be refrafted, without fenfible error, into an an- 

 gle of one degree. But when the angles are larger, they vary 



from this ratio, their fines only preferving the proportion 

 witii accuracy : for example, if the angle of incidence at 

 the fuppofed furface were increaled to 90'', the angle of re- 

 f aftioii would be 30'^ only, inilead of 45^. 



It does not appear that, before Defcartes, any perfon at- 

 tempted to explain the phyfica! caufe of the refradtion, and 

 alio the refleftion of light, which he undertook to do by 

 the rcfoliition ot forces, on the principles of mechanics ; 

 in confequence of which lie was obliged to fuppofe that 

 light pafes with more cafe through a denfe medium than a 

 rare one : thus the ray A B (Plate XVIII. Optics, fg. 10.) 

 falling obliquely on a denfer medium at B, is fuppofed to be 

 aftcd on by two forces, one of them impelling it in the direc- 

 tion A C, and the other in A D, which alone can be aft'efted 

 by the change of medium : and iince, after the ray has entered 

 the denfer medium, it approaches the perpendicular B H, it is 

 plain that this force muft have received an increafe, whiKt 

 the other continued the fame ; for if B E be taken equal to 

 B D, or C A, the confequence of the angle I B H being 

 lefs than ABC will be, that B H muft be longer than B C 

 or AD. 



The firft perfon who queftioned the truth of this explana- 

 tion of the caufe of refraftion was M. Fermat, who allerted, 

 contrary to Defcartes, that light fufTers greater refiftance in 

 water than in air, and greater in glafs than in water ; and he 

 maintained that the refiftance of different mediums, with rc- 

 fpeft to hght, is in proportion to their denfities. M. Leib- 

 nitz adopted the fame general idea ; and they reafoned upon 

 the fubjeft in the following manner. Nature, they fay, ac- 

 complilhes her ends by the (horteft methods ; light, therefore, 

 ought to pafs from one point to another, either by the 

 fhorteil road, or that in which the leaft time is required. 

 But it is plain that the hne in which light paffes, when it falls 

 obliquely upon a denier medium, is not the moft direft or the 

 fhorteft ; fo that it muft be that in which the leaft time is 

 fpent. And, whereas it is demonftrable, that light faUing 

 obliquely upon a denier medium (in order to take up the leaft 

 time poiTible, in paffing from a point in one medium to a point 

 in the other) muft be refrafted in fuch a manner, that the . 

 fines of the angles of incidence and refraftion muft be to one 

 another, as the different facihties with which light is tranf- 

 mitted in thofe mediums ; it follows that, fince light ap- 

 proaches the perpendicular when it paffes obliquely from air 

 into water (fo that the fine of the angle of refradtion is lefs 

 than that of the angle of incidence), the facility with which 

 water fuffers light to pafs through it is lefs than that of the 

 air ; fo that the light meets with greater refiftance in water 

 than in air. This method of arguing from final caufes 

 could not fatisfy philofophers. Dr. Smith obferves that it 

 agrees only to the cafe of refraction at a plain furface ; and 

 that the hypothefis is altogether arbitrary. If he had en- 

 deavoured to accommodate his principle to concave and con- 

 vex furfaces, as he had once propofed, he would foon have 

 perceived its infuificiency. Tlieeafiefl way for a ray to pafs 

 from a given point in any refifting medium into a vacuum, is 

 in a perpendicular to the refracting furface, this being the 

 fhorteft way, through any difficulty or refiftance whatever ; 

 but this being reduced to nothing at the refrafting furface, it 

 may then take any other courfe in vacuo, without any far- 

 ther difficulty ; and, on the contrary, in returning back 

 from the vacuum into tiie denfe medium, it muft take the 

 fliorteft courfe through the fame perpendicular as before. 

 Thus, when the fun fhines upon the atmofphere, all his rays 

 (liould be refrafted into lines tending to the centre of the 

 earth, as being the fhortefl and eafieft way through the at- 

 mofphere, and then we fhould fee the fun exaftly over our 

 heads, in all places and at all times. Such is the ftrange 



confc- 



