QQ METHOD OP EXAMINING DISPERSIVE POWERS. 



coniM of the takes place, depends not only on the denfity of the refiecling 



denier of two prifm, but alfo on the rarity of the medium adjacent to it, the 



extent of that range varies with the difference of the denfities 



of the two media. When, therefore, the refractive power of 



one medium . is known, that of any rarer medium may be 



learned, by examining at what angle a ray of light will be 



reflected from it. 



General faft y 0T inftance, when any object is laid under a prifm of flint- 



aoele is greater, gt a k> w i tn a * r alone interpofed, the internal angle of incidence 



the greater the at which the vifual ray begins to be totally reflected, and at 



fity tf ihe mel whlch the ob J e61 CealeS t0 be feen b > **»i is aboUt 39 ° 



dium towards 10'; but, when the object has been dipped in water, and 



*£ ^ li£ht brou g ht into conta & with the g^afs, it continues vifible, by 



means of the higher refractive power of the water, as far as 



57 \ 9 of incidence. When any kind of oil, or any refinous 



cement, is interpofed, this angle is (till greater, according to 



until total tranf- the refractive power of the medium employed; and, by ce- 



miffion. ments that refract more ftrongly than the glafs, the object may 



be feen through the prifm, at whatever angle of incidence it 



is viewed. 



Fluids areeafily In examining the refractive powers of fluids, or of fufible 



denfe medium f u kft an c es > the requifite contact is eaiily obtained; but, with 



(or glafs) : But folids, which can in few inftances be made to touch to any 



<olids require the eat extent th ; s cannot De effeaed without the interpofition 



interpofition or © / t m r 



a fluid more re- of fome fluid, or cement, of higher refractive power than the 



JJjJJ than medium under examination. Since the furfaces of a ftratum 

 fo interpofed are parallel, it will not affect the total deviation 

 of a ray palling through it, and may therefore be employed 

 without rifk of any error in confequence. 



Thus, refin, or oil of faflafras, interpofed between plate 

 glafs and any other prifm, will not alter the refult. 



A triangular If, on the fame prifm, a piece of felenite and another of 



glafs, pnfm may pi a te-glafs be cemented near each other, their powers may be 

 be ufed for com- r • " r ... r , . . J , 



paringthe powers compared with the fame accuracy as it they were both m ab- 



of two bodies j f l u te contact with it. 



For fuch a mere comparifon of any two bodies, a common 



but a fquare triangular prifm is belt adapted ; but, for the purpofe of actual 



prifm fhews the rneafurement of refractive powers, I have preferred the ufe of a 



Sfciul cilcrtT fq uare prifm, becaufe, with a very fimple apparatus, it (hows 



tion. the fine of refractive power fought, without the need of any 



calculation. 



Figure and ap- j^ ei ^ Fig. 1 . Plate IV. be a fquare or rectangular prifm, to 



plication of the i_« u 



fquare prifm. ^ vhich 



