R E F 



I 'J ohfervc the RefraBkn of a Star, Sic. I. Obfervetlie 

 :aoridiaii altitude of a flar near the zenith ; whence the lati- 

 tude of the place being known, the true declination of the 

 liar iseafily had, the itar being now void of any fenliblc re- 

 fra£lion. 2. Obferve the altitude of the lame liar in any 

 other degree, and note the time by a pendulum. 3. For 

 the given time of obiervation, from the declination of the 

 ftar compute its true altitude. 



This being thus found lefs than the altitude obferved, fub- 

 tra<ft the one from the other ; and the remainder is the refrac- 

 tion for li]at moment, in that degree. 



Refkaction of Altitude, is an arc of a vertical circle, by 

 which the altitude of a ftar is increafed by the refraftion. 



Rekr 'VCTioN cf Declination, is an arc of a circle of declina- 

 tion, by which the declination of a ftar is increafed or dimi- 

 niftied by the refraftion. 



Refraction' cf AfcenfiDtiand Defcenfion,\% an arc of the 

 equator, by which the afceniion and delcenfion of a ftar, whe- 

 ther right or obliq.ue, is increafed or diminiftied by means of 

 the refraftion. 



Refraction of Longitude, is an arc of the ecliptic, by 

 which the longitude of a ftar is increafed or diminiftied by 

 means of the rtfradtion. 



Refraction of Latitude, is an arc of a circle of latitude, 

 by which the latitude of a ftar is increafed or diminiftied by 

 means of refraftion. 



Refrac:'ion, in Commerce, is a term fometimes ufed by 

 merchants, where there has been an overSght in an ac- 

 count, to the prejudice of a perfon ; who thereupon demands 

 reftitution of fo much, added or omitted by miftake. 



You muft make me a refraftion of live pounds forgot in 

 your account. I will deduft or make you a refraftion of 

 30J. charged inadvertently in my bill. 



REFRACTORY, in Chemiftry. See an explication of 

 the term under Apyrous. 



REFRAIN, Fr., in Mupc, the termination of every 

 verfe or ftanza of a fong, by the fame words and the fame 

 melody. (See Burden.) The word, according to Me- 

 nage (Dift. Etym.) is derived from the Spanifti lefran. 



REFRAN, Span., a proverb ; the burden of a fong in that 

 language, being ufually fententious, and including fome 

 moral precept. 



REFRANGIBLE, whatever is capable of being re. 

 frafted. See Refrangibility. 



REFRANGlBILITYo/ /,;>/./, the difpofition of the 

 rays to be refracted. 



A greater or lefs refrangibility is a difpofition to be more 

 or lefs refrafted, in palling at equal angles of incidence, into 

 the fame medium. 



That the rays of light are differently refrangible, is the 

 foundation of fir Ifaac Newton's whole theory of light and 

 coleurs. The truth of the principle, which was inveltigated 

 and ellablifhed by our great philofopher in the year 1616, 

 will appear from the following experiments. 



I. Let EG {Plate XIX. Optics, fg. I.) reprefent the 

 window-fhutter of a darkroom, and Fa hole in it, through 

 which the light is tranfmitted to the prifm ABC, which 

 refrafts it towards P T, where it appears in an oblong 

 form ; its length being about five times greater than its 

 breadth, and exhibiting the various colours of the rainbow. 

 In Jig. 1. ag b reprefents a fecond prifm, which refrafts the 

 light back again to Q, where the image is round ; whereas, 

 without the interpofition of this fecond prifm, the light 

 would have proceeded to P T, and, confequently, have been 

 oblong, as before. In this experiment fir Ifaac Newton took 

 care that the plane a g was placed exatlly parallel to B C, 

 ind alfo ^^ to A C, that the rays might be equally refrafted, 



li K F 



though in contrary wayb, by 1>oth prifuis. He alfo ob- 

 lerved, that both prifms muft be placed very near to one an- 

 other ; for if tiicir dillaucc bo fo great, that colours begin 

 to appear in the light, before its incidence on tlie fecond 

 prifm, thofe colours will not be dcftroycd by the refraftion 

 of that prifm. 



2. Having placed one of two boards behind the prifm at 

 the window, fo that the light might pafs through a fraall hole 

 made in it for the purpofe, and fall on the other board at the 

 dillance of about twelve feet, a hole being made in it to admit 

 the palfage of the incident light, he then placed another prifm 

 behind the fecond board, fo that the light which was tranimit- 

 ed through both the boards might pafs through that alfo, 

 and be again refradtcd before it arrived at the wall. This 

 being done, he took the firft prifm in his hand, and turned it 

 about its axis fo much, as to make the fevcral parts of the 

 image, call on the fecond board, fuccefiively to pafs through 

 the hole in it, that he might obferve to what places on the 

 wall the fecond prifm would refradt them ; and he faw, by 

 the change of thofe places, that the light tending to that 

 end of the image, towards which the refraftion of the firll 

 prifm was made, did, in the fecond prifm, fuffer a refrattioii 

 confiderably greater than the light which tended to the other 

 end. The true caufe, therefore, of the length of that 

 image was difcovered to be no other, than that light is not 

 fimilar or homogeneal, but that it confilts of rays, fome of 

 which are more refrangible than others ; fo that without 

 any difference in their incidence on the fame medium, iome 

 of them fhall be more refratled than others ; and therefore, 

 that according to their particular degrees of refrangibility, 

 they will be tranfmitted through the prifm to different parts 

 of the oppofite wall. 



To make this capital experiment, which fir Ifaac Newton 

 himfelf juftly calls the experimcntum cruets, let S F {Jig. 3.) 

 reprefent a ray of the fun, which, after palling through a 

 hole in the window-ftiutter F, is received by the prifm in 

 ABC, clofe behind which is placed a board D E, with a 

 hole in it at G, to admit any of the rays after they have 

 been feparated by the prifm ; then d e will reprefent the 

 other board, placed at a confiderable diftance from the 

 former, with a hole in it, g, to receive any part of the light 

 tranfmitted through the other board. Behind this fecond 

 board is placed another prifm, a be, through which different 

 rays of light, falling upon it in the very fame place, and with 

 precifely the fame angle of incidence, will be refrafted 

 higher or lower, on the oppofite wall M N. This experi- 

 ment was condufted with the utmoft circumfpeftion and 

 accuracy ; and it is obferved, that neither the different 

 magnitude of the hole in the window-ftiutter, nor the dif- 

 ferent thicknefies of the prifm, at the place where the rays 

 pafted through it, nor the different inclinations of the prifm 

 to the horizon, nor the different matter of the prifms, made 

 any fenfible change in the length of the image. 



3. In order farther to eftablifit this famous hypothefis of 

 the different refrangibihty of the rays of light, he held a 

 prifm in a beam of the fun, which was tranfmitted into the 

 room through a hole in the window-ftiutter, fo that its axis 

 might be perpendicular to that beam ; and having turned the 

 prifm about its axis to make the image afcend and defcend, 

 and when it feeraed to be ftationary between thefe contrary 

 motions, he fixed the prifm fo that the refrattions of both 

 fides might be equal to each other. In this utuation he 

 looked at the hole through the prifm, and obferved the length 

 of its refrafted image to be many times greater than its 

 breadth : the moft refrafted part of it was violet, the leaft re. 

 frafted red, the middle parts blue, green, and yellow, in 

 order. The fame thing happened when he removed the prifm 

 4 G 2 out 



