D:Jpirftvt Powers of Fluid Mediums. J. 



frscled image appears colourlefs, without regarding the coincidence, the ratio of the mean re- 

 fra£live and difperfive powers of that kind of fluid, and that kind of glafs, with which the ex- 

 periments are made, will be obtained from the angles of the pilfms being given in both cafes. 



In order to afcertain the abfolute refraftive denfity of glafs, or any other medium, that 

 is to fay, the general ratio of the fines of the angles of incidence to the fines of the angles 

 of refraftion of the mean refrangible ray which obtains in that medium, tlie Doftor took a 

 Jire£l method, fimilar in principle to that employed by Sir Ifaac Newton, and defcribed by him 

 in the feventh propofition of the firft book of his Optics, and likewife in his Optical Leftures, 

 p. 54; but which, as he juftly remarks, will be found much eafier, and perfeftly accurate. 



Inftead of caufing the rays to pafs through the fights of a large and accurate quadrant at 

 the diftance often or twelve feet, as direcled by Sir Ifaac Newton, the Doftor employed a 

 Hadley's quadrant, in^the following manner : 



Plate I. fig. I — I reprefents the index-glafs, and II the horizon^glafs of a Hadley's quadrant. 

 SI reprefents a folar ray iilcident on the in<tex-glafs, thenfce refleclc-d to the horizon-glafs H, 

 and from it to the eye at E. The line /g reprefents another folar ray, incident on the 

 prifm P, and through it refra£led to the eye at E. When tlie prifm is turned flowly round 

 its axis till the fpectrum G appears at its greateft height, this is its proper pofition. The 

 angle formed by the direiSl and refracted ray is than the leaft polTible, and the angles of 

 incidence and emergence are equal. Let the prifm be fecured in this pofition. A flight 

 infpeftion of the figure will flievv, that when the reflefted and refrafted images of the fun 

 are made to coincide, the angle marked by the index of the quadrant is the fame which the 

 incident ray fg forms with the refracted ray PE produced. For SZH is the angular dif- 

 tance of the fun, and his doubly reflefled image marked by the index ; and the angle/^ G, 

 which the ray incident on the prifm forms with the refradled ray produced, is equal to it j 

 fg and SI being parallel, and PZ and HZ being coincident. 



The manner in which the ratio of the fines of the angles of incidence and refra£lion may 

 be computed, from the above angle and the rcfrafting angle of tlie prifm being given, is 

 fully explained in the celebrated works which have juft been quoted. 



The Doftor here remarks, that as it is the ratio of refra£lion of the mean refrangible 

 ray which is wanted, the centre of the reflefled image of the fun ought to be made to coin- 

 cide with the centre of the coloured fpeclrum, as reprefented in the figure ; and if, inftead 

 of this, the coincidence be formed with the mofl or leaft refrangible ray, or any of the in- 

 termediate rays, it will be the ratio of refra£lion of thefe rays, and not of the mean refran- 

 gible ray, which will be found from the obfervation. Hence this mediod might be prac- 

 tifcd for determining the difperfive power, as well as the mean refraiSlive denfity of any 

 tranfparent fubftance, whether folid or fluid -, but the Doflor has preferred a combination 

 of prifms or lenfes, becaufe it is the relative ratios, more than the abfolute ratios, which are 

 inoft immediately wanted. 



By this prifmatic apparatus, the optical properties of a great variety of fluids were exa- 

 mi!ic<l. The folutio)is of metals and femi-mcials proved in all cafes more difperfive than 

 crown-glafs. Some of the falts, as, for example, fal-aninioniac, greatly incrcafed the dif- 

 perfive power of water. The marine acid difperfes very confiderably, and this quality in- 

 creafcs with its ftrength. The moft difperfive fluids were accordingly found to be thofe in 

 which this acid au<l the metals were combined. The chemical preparation called caii/lui/m 



15 2 antiinoriia/f, 



