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VOL. XXVII.] PHILOSOPHICAL TRANSACTIONS. Q\f 



same. The object is a piece of white paper, in form of a cross, pasted on a 

 black board, and is fixed at the end of the moving limb, which is about 7 feet 

 long from the sight, its diameter is about 2-i- inches, which just comprehends 

 the sight through the slit; so that when the object is wholly within view, we 

 conclude the observation to be exact. With this apparatus the experiments are 

 made as well by candle-light as day-light, the presence of the sun beams being 

 nowise necessary; and I think they may be depended on as pretty accurate. I 

 have taken the specific gravity of the several liquids, where I could obtain a 

 sufficient quantity, as appears by the table; so that if any person should have 

 the curiosity to repeat these experiments, he must expect a different angle of 

 observation, if the specific gravity agree not with the table; for sometimes it 

 happens, that liquids of the same dfenomination are not always of an equal 

 goodness, and consequently will have a different specific gravity and refraction. 

 The crystalline humour of the ox eye I pressed into the angle of the prism, 

 by which it received its form, and gave the angle of observation, as specified in 

 the table. I could not see the common object through it, but was forced to 

 make use of a candle for that purpose; the flame of which appeared very broad, 

 at least 5 or 6 inches, nearly in the form of a half moon ; but what should 

 occasion such a change of figure I cannot at present determine. Of all the 

 fluids I have tried, I find nothing to refract a ray of light less than water ; yet 

 there are several other liquids which make the same angle. I observe oil of 

 bees- wax to be the lightest fluid, and butter of antimony per deliquium to be 

 much the heaviest; the difference of specific gravity between these two bodies 

 is as 662 is to 1976, that is, nearly as 1 to 3: and the ratio of their refractions 

 only as 10000 is to 6885 bees- wax, so is 594 1 antimony to the same radius, 

 that is, nearly as 1 to 1.1 6. Likewise oil of vitriol is in specific gravity to oil 

 of sassafras, as 1510 is to 898; yet the ratio of refraction of the lightest is most 

 considerable, being in proportion as 10000 is to 64/5 sassafras; so is the same 

 radius to 7011 vitriol. Thus I find, that a body does not refract in proportion 

 to its specific gravity, but from some quality peculiar to itself, whether it be 

 from its inflammability, or from any different texture, or figure of its compo- 

 nent parts; or whatever else it be, I shall, with the application of these expe- 

 riments, submit to this honourable society to determine. 



VOL. V. 4 K 



