44 Dr Brewster and Dr Gordon on the Human Eye. 
four luminous sectors like the crystalline lens of quadrupeds. 
The human cornea produced a double refraction of an opposite 
kind to that of the crystalline, and the iris, which is semitrans- 
parent, gave the same phenomena as the crystalline. 
Since the preceding notice was read, M. Chossat of Geneva ^ 
has performed a very extensive series of experiments on the 
refractive powers of the humours of various animals. His 
results for the human eye confirm those which we have given 
above, and, as they were obtained by a different mode of observa- 
tion, it can no longer be doubted that the two humours of the 
human eye have a higher refractive power than water ; and that 
the refractive density of the vitreous exceeds that of the aqueous 
humour. 
The following is a comparison of our observations with those 
of M. Chossat: 
Aqueous humour, 
... 1,3366 
Chossat’s Results. 
1.338 
Vitreous humour, 
... 1,3394 
1.339 
Outer part of crystalline, — 
... 1,3767 
1.338 
Middle Coats of ditto, 
... 1,3786 
1.395 
Central part of ditto, 
... 1,3999 
1.420 
Whole crystalline, 
... 1,3839 
1.384 Mean. 
The experiments of M. Chossat give a much greater variation 
of refractive density to the crystalline humour, though the 
mean of his three measures is almost exactly the same as that 
wliich we obtained for the action of the whole crystalline. I 
have no hesitation in considering our result as the most accu- 
rate, not only from the precautions which were taken to avoid 
error, but from the improbable circumstance that the first coat 
of the crystalline lens should be inferior in refractive power to 
the vitreous humour, and from the coincidence of our result 
for tlie central part of the crystalline with that obtained by Dr 
Thomas Young, in his able dissertation on the mechanism of the 
Human Eye "[*. He makes the ratio of the refractive power of 
* M. Chossat’s Experiments were published in the Bulletin des Scieyices, par la 
S r. Philomath, for Jun • 1818, p. 94, ; and in the Bibliotheqzie UniverscUe for 
l^ppteuiber ISIS, p. 26. 
t rUil. Trans. 1801, cr Nat. Phil. Vol. ii. p. 5S0. 
