ON THE REFRACTIVE INDEX OF FLUIDS. Lis 
was published by Bertin himself in the ‘Ann. de Chimie et 
de Physique,’ 1849, xxvi, p. 288. To determine the refrac- 
tive index of a plate of glass, he proceeds as follows. A 
micrometer is used as an object, and its amplifications ob- 
served—first, as it lies on the glass plate; second, as it lies 
beneath it; and, third, as it rests on the stage without the 
interposition of the plate. In the course of these observa- 
tions the olject-piece must remain a fixed point, and the 
necessary motion be given to the eye-piece only. The suc- 
cessive amplifications, in the above order, are termed G, y, g, 
and the formula for finding the index of refraction is 
ene od 
meet Gly 
When the plate is very thick, it is better to compare it with 
another whose index is already known. Then, 
( -) i 1 
Saleh Sse eee 
This method, like all the others which we have had occasion 
to describe, is also applicable to fluids. It is said, that its 
possible error cannot exceed 1 in the second place of deci- 
mals. 
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aaa 
py 
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Sle 
ea 
S 1 
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be 
Note.—Of the comparative value of these four methods 
I have had no opportunity of judging; but on applying 
the second to the determination of the refractive power of 
water, turpentine, castor oil, and other fluids, I have found 
its results very uniform and satisfactory. (Trans.) 
