16 On the UNE QUAL 
flint-glafs, to great irregularities in its denfity, difcoverable by e 
ftreams of light, like comet’s tails, 1ffuing in different directions 
from the difc of Venus, which was the planet obferved. By 
fhaking 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. 
Ir 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 ether, into which a {mall 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 fubject. : 
But as folutions of faline fubftances in this diluted ftate do 
not differ materially in difperfive power from the effential oils, 
thefe two kinds of fluids may be ufed indifferently. 
THERE is, however, a particular cafe, in which water or vi- 
triolic «ther, impregnated with antimony or mercury, will 
have the advantage, from being lefs denfe than effential oils ; 
and that is, where it is required to produce a fingle refrac- 
tion, in which there fhall be no difference of refrangibility of 
heterogeneal light. As this expreffion 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 
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