REFRACTION. 



be compared with the fame accuracy as if they were both in 

 abfoliite contaft with it. 



For fuch a mere comparifon of any two bodies, a com- 

 mon triangular prifm is befl. adapted ; but, for the purpofe 

 of aftual meafurement of refraftive powers. Dr. WoUalton 

 has preferred the ufe of a fquare priim, becaufe, with a very 

 fimple apparatus, it ftiews the fine of rt-fraftive power fought, 

 without the need of any calculation. 



Let A {Pl"t': XVITT. Optics, fig. 12.) be a fquare or rec- 

 tangular prifm, to which any fubllance is applied at b, and 

 let any ray of light parallel to c h be refraded through the 

 prifm, in the direftion bde. 



Then, if f /■ and edhfi taken proportional to the fines 

 that reprefent the refraftive powers of the prifm and of air, 

 fg, which is intercepted between / and the perpendicular 

 eg, will be the correfponding fine to reprefent the refractive 

 power of the medium b. For fince edg (oppofite to cf) 

 is the angle of refraftion, efg (oppofite to e d) nnift be 

 equal to the angle of incidence bdh; and ej : fg - b d : 

 dh :: fine al cbi : fine of h b d. 



All, therefore, that is reqiiifite for determining the rc- 

 fraftivc power of h, is to find the means of m.eafuring the 

 line fg. On this principle, the inflrument in fg. 13 is 

 conltrufted. On a board, ab, is fixed a piece of flat deal 

 cd, to which, by a hinge at d, is jointed a fecond piece 

 de, 10 inches long, carrying two plane fights at its ex- 

 tremities. At e is a fecond hinge, connefting ef, IJ.83 

 inches long; and a third at the other extremity of e f, 

 by which fg is conneAcd with it. At i alfo is a hinge, 

 uniting the radius ig to the middle of ef; and then, fince 

 g moves in a femicirclc cgf, a line joining e and g would be 

 perpendicular to fg. 



The piece cd has a cavity in the middle of it, fo that, 

 when any fubftance is applied to the middle of the prifm P, 

 it may continue to reft horizontally on its extremities. 

 When ed has been fo elevated, that the yellow rays in the 

 fringe of colours (obfervable where perfeft refleftion ter- 

 minates) ai-e feen through the fights, the point g, by means 

 of a vernier which it carries, fhews by infpcftiou the length 

 of the fign of refraflion fought. 



The advantages which this method pofleffes above the 

 ufual mode of examining refraftive powers, are greater than 

 they may at firft fight appear. The ufual praftice has 

 been, to form two furfaces of the fubftance under examina- 

 tion, fo inclined to each other that the deviation occafioned 

 by them might be meafured. The inclination of thefe fur- 

 faces to each other muft alfo be known ; and thence the re- 

 fraftivc power might be computed. But, in the method 

 here propofed, it is fufficient to have only one furface, and 

 the refult is obtained at once, without computation. 



The facility of deterniining refraftive powers is confe- 

 quently fuch as to render this property of bodies a very 

 convenient teft in many philofophical inquiries. 



The following table exhibits a feries of fubftances, ar- 

 ranged according to their refraftive powers. 



Diamond - - 



Plumbago . . . 



Native fulphur (double) 



Glafs, confifting of lead 6 and fand 1 



Glafs of antimony 



Jargon . . . 



Spinelle ruby 



Arfenic 



Muriate of antimony, variable. 



White fapphire 



Gum dragon 



2.44 



2.04 

 1.987 

 1.98 

 1.95 

 1. 812 

 1. 811 



1.768 



Iceland fpar, ftrongeft 

 Sulphate of barytes (double) 

 Balfam of Tolu .... 



Guaiacum ... 



Benzoin - - . - 



Flint glafs - ... 



Ditto .... 



Horn - - . 



Phofphoruf - - - 



Mica .... 



Opium - . - - 



Amber - . - 



Rock cryftal (double) 



Old plate-glafs - - - 



Colophony - - . 



Box-wood - - - 



Bees-wax . . - 



Ofi of fali'afras . - . 



Red fealing-wax - - 



Spermaceti, cold 



Sugar, after fufion - - 



Arfeniate of potafh 

 Maftic 



Elemi .... 



White wax (cold) - - - 



Oil of cloves ... 



Copal .... 



Anime ... 



Radcliffe crown-glafs ... 



Pitch .... 



Centre of cryftalline of fifti, and dry cryftalline 1 

 of an ox - . -J 



Can?.da balfam . - - 



Crown glafs, common 

 Selenite . . 



Caoutchouc . - . 



Gum lac . - . 



Dutch plate-glafs ... 



Human cuticle . . 



Gum arabic - - 



Balfam of capivi ... 



Oil of amber ... 



Enghfli plate-glafs . . . 



French plate-glafs ... 



Oil of nutmeg ... 



Sulphate of potafti - - . 



Tallow, cold ... 



Iceland fpar, weakeft 



Camphor ... 



Linfeed oil - - . 



Butter, cold - - 



Effence of lemon ... 



Ou of turpentine, common 

 Oil of turpentine, reftified 

 Oil of almonds . . 



Oil of ohves . . - 



Oil of peppermint - - - 



Oil of lavender . . - 



Tallow, melted 

 Alum 



Spermaceti, melted - - . 



Cryftalhne lens of an ox 



to . . - 



Computed average of ditto 



Sulphuric acid - . - 



Fluor fpar . ... 



Nitric acid (fp. gr. 1.48) 



1.657 

 1.646 

 1.60 

 1.596 



1.586 

 1.583 



'•579 



'•547 

 '•547 

 '•545 

 1-543 



1.542 

 '•536 



'•535 

 '•535 

 '•535 

 '•533 



•53c> 



.528 

 .525 

 .525 

 .524 



•517 



.514 



.507 



.505 



.504 



.500 



•497 



•495 



•49 



.488 



1.487 



.485 



1.480 



i.476 



1.476 



.470 



i.469 

 .468 



1.467 

 .460 



•457 

 .446 



•447 

 1.380 



•430 

 •435 

 •433 

 .410 

 Alcohol 



