16 On the UNEQUAL 



flint-glafs, to great irregularities in its denfity, difcoverable by 

 ftreams of light, like comet's tails, iffuing in different directions 

 from the difc of Venus, which was the planet obferved. By 

 making the object-glafs, thefe might be, in a great meafure, 

 removed, but foon returned ; and after ftanding all night, 

 broad veins, in different parts of the included fluid, were per- 

 ceptible to the naked eye. 



It was neceflary on this account to reject very denfe fluids. 

 The antimonial preparation I found might be reduced to a fuf- 

 ficient degree of fluidity, by mixing it with fpirit of wine or 

 vitriolic aether, 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 crude fal ammoniacum, may be employed for producing 

 refraction without colour, and without being fubject to that 

 irregularity of denfity to which flint-glafs, and very denfe dif- 

 perfive fluids, are fubjecl:. 



But as folutions of faline fubflances in this diluted ftate do 

 not differ materially in difperfive power from the eflential oils, 

 thefe two kinds of fluids may be ufed indifferently. 



There is, however, a particular cafe, in which water or vi- 

 triolic aether, 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 Angle refrac- 

 tion, in which there fhall be no difference of refrangibility of 

 heterogeneal light. As this expreflion may found ftrange in the 

 ears of opticians, I fhall, before proceeding farther in the ap- 

 plication of the experiments which have been recited, explain 

 what is meant by it. 



Cafes 



