t v? I Royal §c wty nf. London,^ 



prifuratuj • fpeculnm employed- by Sir Ifaac Newton in his 

 refloftuag *etefcopc.. The angle at which the total reflection 

 of light of any kind firft takes place at the fnrface of a rarer 

 medium depends on the comparative denfity of the two me- 

 diums, in contact, and hence the meafurement of this angle 

 ivadiiy favnifhes a determination of the ratio of refraction at 

 the common fnrface for the kind of light oblerved. Thus by 

 ineans of a triangular prifm, a drop of each of two or more 

 fluids being placed fide by fide on the utoder fnrface, it may 

 eafily be; found, by inclining the prifm more and more, which 

 of the dark -fpots.ii rfti difappears, and it follows that the re- 

 flective fluid has the weakett refractive power. But when a 

 folid is examined, it mull in general be united by the inter- 

 portion of fome fluid of a -higher refractive denfitv, other-wife 

 the contact will be too imperfect 5 and it is eafilv ; mown that 

 this interpofition does not affect the ultimate reft* It.. Eut 

 fbf determining at once the numerical ratio of die fines, Dr. 

 Wollafton.has. 'invented miiapparatus where, bv Inferos: of a 

 rectangular prifm of. flint gtals, i tl>e index of Vefra6bion»of 

 each fubftance is read off at once by a vernier, the three fides 

 erf a moveable triangle performing the operations of reduction 

 of the ratios in a very compendious manner. In this meJiod 

 k-is obvioufry unoeceiTary that thcfuMances to he cxanvned 

 fhould be of any determinate forme ;and it is as eafy to afcei - 

 tht refractive denfity of the-mio>ft<ipaque as of themoft traFif- 

 parent bodies, provided they be fei^'refractive than the .prifm 

 employed. It may alfo fervc as a cheinical teft, for. example 

 in eiTentia-1 oils, which when adulterated are generally reu^ 

 dered lefs Tefra&ive ; and a very minute quantity is funic icnt 

 for the experiment. Where the medium is of variable den- 

 fity, this is aim-oft the only : mode in which its rcn active power 

 can be afcertained ; hence it is of 'Angular utilitv in exa- 

 mining the refraction of the eryhu-iluie lens. (Philofbphical 

 Tranfaclions, -1801, p. 41.) A'copious table of the refrac- 

 tive powers of vai ions fubfbnces is here inferted. Thedi- 

 foerfive powers of difierent fubftauccs are inferred from fimilar 

 obfervaiions upon the fringes which ufually accompanv, or 

 rather conftitute, the boundary of reflection : the author ob-» 

 ■ s that they are fometimes wanting, or even reverted, 

 w-he*i the difperfion is equal at difierent angles of deviation,- 

 .or when it is greater even with a lets deviation, as when oil 

 of faflkfras is applied to a prifm of flint glafs, as well as in 

 many, cafes oftfpars with fluids. Solutions of metallic fa Its 

 in gen-eraJ-aye found to be verv highly 4ifperfive : by weaken- 

 ing the folution till the line of reparation became eoloudefs, 

 and then noting the refraclive denfity, Dr. Wol 1 alio 11 has 



been 



