REFRANGIBILirr of LIGHT. 63 



differ in difperfive power, I was informed, that fomething of the 

 fame kind had been obferved by fome foreign philofophers, smd 

 in particular by the celebrated M. Clairaut and M. Bosco- 



VICH. 



The obfervation of the former appears in a Memoir of the 

 French Academy of Sciences, of fo old a date as the year 1757. 

 As the paffage relating to this fubject is fhort, and does great cre- 

 dit to the author, as an accurate obferver of the refults of ex- 

 periments, I mail beg permiffion to tranfcribe it. " II y a en- 

 " core un fait important que nos experiences nous ont appris, 

 " c'eft que les corrections des iris faites par les prifmes combi- 

 " nes, ne font jamais audi parfaites qu'on le croiroit d'apres 

 " les termes de M. Dollond. Dans le cas du prifme de verre 

 " place dans l'eau ; par example, apres avoir fait varier les 

 " plaques qui determinent Tangle du prifme d'eau, jufqu'au 

 " point ou les objects vus a travers les deux prifmes, ne paroif- 

 " fent point decolores, du moins aux vues ordinaires, on trouve 

 " en pla^ant ces prifmes dans la chambre. noire, qu'il refte tou- 

 " jours quelque petit limbe de couleur vers les bordes de 

 " l'image du foleil, ce qui vient fans doute de ce que les par- 

 11 ties du fpeclre que chaque matiere refringente donne, ne font 

 " pas exaclement proportionelles aux longueurs totales de ces 

 " fpeclres. Mais ces inegalites qui deminuent a mefure que 

 f i les angles des prifmes font plus petits, doivent etre comme 

 " infenlibles dans le cas des lentilles adoffees, vu la petitefTe des 

 " angles de refringence qui ont lieu alors,'' I fhall only re- 

 mark on this paiTage, that M. Clairaut would have obferved 

 the uncorrected colour better, if he had made ufe of a much 

 fmaller pencil of light than he appears to have done, and would 

 not have concluded fo haftily, that this uncorrected aberration 

 was of little confequence to the performance of telefcopes, if he 

 had recollected, that the fmallnefs of the angles of the lenfes is 

 greatly overbalanced by the magnifying power of the eye- 

 glafs. 



M. 



