Difpi'i-Jtve Powers of Fluids. e 



ing to expe£latioii, the colour was found to be removed. But the Do£l:or was furprifed to 

 find, on directing the inftrument to a planet, and ufing a deep eye-glafs, that this fluid, in 

 its highly concentrated ftate, was fubjeft, like flint-glafs, to great irregularities in its denfity, 

 difcoverable by flreams of light, like comets' tails, iffuing in differcMit diredions from the 

 difk of \'enus, which was the planet obferved. By fliaking the objecl-glafs, thefe might be 

 in a great meafure removed, but foon returned; and, after (landing all night, broad veins, 

 in different parts of the included fluid, were perceptible to the naked eye. 



It was neceflary, on this account, to rejed: very deiife fluids. The antimonial preparation 

 was found to be reducible to a fufficient degree of fluidity, by mixing it with fpirit of wine 

 or vitriolic ether, into which a fmall quantity of the marine acid had been previoufly dropped. 

 This prevents any precipitation of the femi-metal in the form of a calx. In this diluted 

 form, either this preparation, or the folution of corrofive fublimate mercury alone in fpirit 

 of wine or in water, with the addition of cxaAtfnl nmnwnincum, may be employed for pro- 

 ducing refraction without colour, and without being fubjecl to that irregularity of denfity 

 to which flint-glafs and very denfe difperfive fluids are fubjeift. 



But as folutions of faline fubftances in this diluted ftate do not differ materially in dif- 

 perfive power from the eflentlai oils, thefe two kinds of fluids may be ufed indifirrently. 



In one cafe, however, our author remarks, that water, or vitriolic ether, impregnated with 

 antimony or mercury, will have the advantage from being lefs denfe than eflential oils, and 

 that is, where it is required to produce a fingle refraftion, in which there fliall be no difl^er- 

 ence of refrangibility of heterogcneal light. As this expreflfion might found flrange in the 

 ears of Opticians, the Doclor has employed a confiderable part of his paper to explain what 

 is meattt by it. He {hews, that there are cafes of fingle refraftion in which the violet rays 

 are the leajl refrangible ; or in which all the rays are equally refrangible ; or in which the red 

 rays are refrafScd from the perpendicular, and the violet rays towards the perpendicular, while the 

 ?nean refrangible ra\s fuffer no refraclion, 



Thefe pofitions muft, no doubt, at firfl: confideration, appear paradoxical ; but their fin- 

 gularity will vanifli on attending to two circumRances: ift. That thefe refraftions, though 

 fingle, are not efl^cflcd by the fimple agency of one medium on the confine of a vacuum, but 

 by the difl^erence of the contrary actions of two mediums at their cominon furface ; and, 

 adly, That Opticians, from the habit of contemplating the crown and flint glafles, have 

 aflbciated the notion of a greater difperfi\ e power with that of a greater power of mean 

 refraction. — But our author, having afcertained various fafts in which the greater power of 

 difperfion accompanies the lefs mean refractive power, has in confequence ihewn that 

 an achromatic lens may be conltru£ted, in v/hich all the refractions are made in the fame 

 direction *. The cafes of fingle refraction here mentioned are very perfpicuoufly and at 

 fomc length flicwn, with fuitable diagrams, as well from the general facts before enumerated, 

 as from aflumed powers of attraction of the mediums on the rays of light, after the fup- 

 pofition of Sir Ifaac Newton. For the fake of brevity, however, 1 fliall give the fubftance 

 of his explanation, without particularly attending to the order of arranj^eincnt. 



• In an objcfl-gtafs formed vf oil of lurpeotioe inclu<lcd between two double cnnvex lenfcs, tlic ladii of 

 wh'ili. convexities arc as fix ro one, and the deep fides inwards, tlicit are four tcfrnfllons, all towards the axis j 

 and the aberration, from difference of refrangibility, is reinovcd. This componnd lens has twenty inches focal 

 Itngth, and one inch and half aperture. ■ Its performance as a tclelcopc is not cLntfiiiptiblc. B. 



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