.'METHOD OF EXAMINING DISPERSIVE POWERS. 91 



which any fubftance is applied at b, and let any ray of light pa- 

 rallel to cb be refracted through the prifm, in the direction bde. 



Then, if ef and ed be taken proportional to the fines that Conftruction to 

 reprefent the refra&ive powers of the prifm and of air, fi^ZtoZfifr? 

 which is intercepted between /and the perpendicular eg, will any given rarer 

 be the correfponding fine to reprefent the refractive power of bod y int0 S ,afs » 

 the medium b. For, fince edg (oppofite to ef) is the angle 

 of refraction, efg (oppofite to ed) muft be equal to the angle 

 of incidence bdh; and ef i fg : : b d : d h : : line of c b i : 

 fine of h b d. 



AH therefore that is requifite for determining the refraclive instrument 

 power of h, is to find means of meafuring the line fg. One iv j n g th * fin * 

 this principle, the inftrument in the annexed fketch (Fig. 2.) yinpe 

 is conftrucled. On a board ab is fixed a piece of flat deal c d, 

 to which, by a hinge at d, is jointed a fecond piece de, 10 

 inches long, carrying two plane fights at its extremities. At 

 e is a fecond hinge, connecting ef 15,83 inches long; and a 

 third at the other extremity of ef by which fg is connected 

 with it. At i alfo is a hinge, uniting the radius ig to the 

 middle of ef; and then, fince g moves in a femicircle egf 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 e d has 

 been fo elevated that the yellow rays in the fringe of colours 

 (obfervable where perfect reflection terminates) arefeen through 

 the fights, the point g, by means of a vernier which it carries, 

 (hows by infpection the length of the fine of refraction fought. 



The advantages which this method poflefles above the ufual This method i s 

 mode of examining refractive powers, are greater than they^ e jr Cl !" te 

 may at firft fight appear. The ufual practice has been, to form requires only on© 

 two furfaces of the fubftance under examination, fo inclined f " r & c e and gives 

 to each other that the deviation occafioned by them might be fight, 

 meafured. The inclination of thefe furfaces to each other 

 muft alfo be known ; and thence the refractive power might 

 be computed. But, in the method here propofed, it is fufn- 

 cient to have only one furface, and the refult is obtained at 

 once, without computation. 



The facility of determining refractive powers, is confe- its facility ren- 



quently fuch as to render this property of bodies a very conve- ders ' : \ ufe , ful ** 



• o- ,.,/-,•%..■ -r. it • ,i a teft in philofo- 



nient teft in many philolophical inquiries, ror dilcovering tnephi ca i inquiries, 



purity of eflential oils, fuch an examination may be of con- 



2 fidcrable 



