112 ON THE REFRACTIVE INDEX OF FLUIDS. 
1. Aqueous Humour from Cow’s Eye. 
By measurement No. l - m=1°3495 
” » ify \eapaleaam 
» 13494 
», 1°3496 
3, 1°38465 
OE ) 
By mean ..,. + *osto-gf=-djoaen 
Extreme difference of measures . . = 0:0039 
Probable error of mean = 0:0005 
2. Vitreous Humour of same Eye. 
By measurement No.1] . n=13412 
5B) » 2 »” 13421 
” » O 5B) 13474 
” be ig Fite, Wes 
»” »” 5 ” 1°3426 
By mi¢an:) © 92) Sy sed Sy a date 
Extreme difference of measures . . = 0:0062 
Probable errorofmean. . . . . == 00-0007 
III. Moser’s Method. 
An account of this method is contained in the ‘ Reper- 
torium der Physik,’ v, p. 395. Moser uses an object-piece 
of long focus, taken from a common reading-microscope, and 
fixed at one end of a tube of at least fourteen inches in 
length, to the other extremity of which an eye-piece is 
adapted. The refractive index of a transparent plate with 
parallel surfaces, or of a layer of fluid, is then found by the 
] 
following formula: x=? (1 —*), When an object is 
brought into focus, and the refractive medium then inter- 
posed, the tube must be lengthened, or rather the object- 
glass withdrawn to a certain distance, in order that a distinct 
view of the object may be obtained. The difference between 
these two focal distances is then called 2, the thickness of 
the interposed refractive medium is termed 7, and the index 
of refraction = 7. 
IV. Bertin’s Method. 
This method was communicated to the French Academy, 
through Regnault, in April, 1849, and a full account of it 
