4 Eyperimtuts on tht RefraBrut nnd 



antimenialf, or hulyruiii an/imonii, in its mod concentrated ftate, when it has juft attracted 

 fufficient humidity to render it fluid, poflcfles the quality of dirperliiig tiic rays in fuch ai» 

 alloiiilhing degree, that three wedges of crown-glafs are nccetlary to remove tlie colour 

 produced by one wedge uf this fubdancc, of an equal rcfrading angle oppofed to them. 

 The great quantity of the fenii-nietal retained in folution, and the highly concentrated ftatc 

 of the marine acid, arc confidered hy the author as the caufe of this fcarcely credible effect. 



Qorrofivc fublimate mercury, added to a folution of fal rnmnonwcuni in water, pofl<;ll'es 

 the next place to the butter of antimony among the difperfive fluids which he examined. 

 It may be made of fuch a degree of fircngth as to require a wedge of crown-gbfs of double 

 the refracting angle, to remove the colour which a prifm of it produces. The mercury and 

 marine acid contained in this folution arc manifeflly the caufe of its difperfive power : for 

 neither the water, nor the volatile alkali, wliicli are its other component parts, will be found 

 capable, if tried feparately, of contributing towards lliis efl'ccl. 



The effential oils were found to hold the next rank to metallic folutions, among fluids, 

 which polTefs the difperfive quality. The mofl. difperfive were found to be thofe obtained 

 from bituminous minerals, fuch as the native pctrolea,pitcoal, and amber. When the re- 

 fraction is without colour, the proportion of the refracting angle of a prifm of tliefe, to 

 the refratling angle of a prifm of crov«n-glafs a£ling in oppofition, is about two to three. 

 The difperfvve power of the eflcntial oil of faflafras, is not mucli inferior to thefe. The 

 eflential oil of lemons, when genuine, requires the refra£ling angles of the prifms neceflary 

 to produce a colourlefs refradion, to be as three to four. In oil of turpentine, this pro- 

 portion is as feven to fix; and the elfential oil of rofcmary is ilill Icfs difperfive. 



Some expreflcd oils, which were examined, were found not to differ fenfibly in difperfive 

 power from crown-glafs-, which was alfo the cafe wrtbredified fpirlts, and with nitrous and 

 vitriolic ether. 



Having been thus fuccefsful beyond his hopes in difcovering fluids capable of removing 

 die great imperfcclion of tclefcopes arifiiig from the different refrangibility of lights, the 

 next object of the author was to fcle£l from this variety thofe that feemed bell adapted to 

 optical purpofes. 



There was no doubt that fhofe mediums which moll difperfe the rays were cittcris pa- 

 ribus to be preferred, k will alfo be found, when the method of correfting thofe errors 

 which arifc from the fphcrical figures of lenfes comes to be confidered, that there is ap- 

 parently an advantage in ufing a difperfive medium, whofc mean refractive denfity excetda 

 the mean refractive denfity of crown-glafs. 



As the antimonial cauftic poflcfles both thefe advantages in a degree far beyond wiiat 

 was to be exptdled in any fluid, fome of it was included between two double convex lenfes 

 of crown-glafs, whofc radii of convexity were as two to one. The lead convex fides of, 

 thcfc were turned toward each other, and they were kept at a proper diftance by means 

 of a glafs ring. The cavity was then filled with the ftrongcft butter of antimony. li.re 

 it is evident, that there is a coiKave lens of the difperfive fluid adting ui oppofition to the 

 two convex lenfes of erown-glafs, and that the proportion of the radii of thefe is the l.ime 

 which was found by the prifms to correct the colour, namely, three wedges of crowr.-glafs 

 to one of tlis butter of antimony. 

 This compound objeCt-glals being put into a tube, an cje-g-lafs was applied, and, accord, 



i"6 



