14 Transactions of the Royal Microscopical Society. 
transparent mineral jQlling a cavity in an old lava of Vesuvius with 
fresh soft balsam, having an index of about 1 • 44, I found that 
the index of the mineral was about 1 '49, which closely corresponds 
with that of analcime, or some other analogous zeolite. 
Very frequently in large sections of rocks some interesting 
object may occur so far from the edge that direct comparison with 
the balsam might lead to serious error. This difficulty may be 
overcome by comparing it directly with some other mineral the 
index of which is either known or has been previously determined. 
For this purpose quartz is eminently suitable, since its mean 
apparent index, which is about 1 * 55, does not vary materially ; 
it is so frequently met with, and so easily identified. By thus 
comparing with quartz a reddish mineral having no double refrac- 
tion, met with in a porphyritic rock from Naddle Fell, I found 
that its index was about 1 • 82, which clearly indicates that it is a 
garnet containing much iron. 
To be able thus to determine the index from such thin sections, 
and from portions of minerals with no solid transparent part more 
than 2 I 0 i^ch in diameter, will for the future make it 
possible to identify the mineral constituents of rocks in a far more 
satisfactory manner than heretofore. In order to obtain such good 
results as those described from very thin sections, it is, however, 
necessary to take the means of many observations, and thus 
eliminate the unavoidable errors of individual observations, 
I have so far considered almost exclusively the mean indices of 
refraction and not the separate indices of uniaxial and biaxial 
crystals. I do not propose to enter at large into this part of 
my subject, but still I think it is not desirable to omit it alto- 
gether. 
The meteoric irons of Krasnojarsk and Kittersgriin contain a 
clear transparent mineral which has been proved to be olivine by 
the usual methods. I subjoin the values of the three indices, and 
give for comparison those of olivine, according to Des Cloiseaux : 
Krasnojarsk 1*71 1'60 1*68 
Rittersgriin 1*70 1'68 1-66 
Olivine 1-70 1-68 l'G6 
Such a close agreement clearly shows that the methods I have 
adopted are correct, and give very satisfactory results when the 
section is not too thin, and a sufficient number of observations made 
to eliminate accidental errors. 
In the case of the black augite of the lava of Vesuvius I found 
that the three indices were about 1’80, I '76, and 1’75. This 
proves that there are two optic axes, and that the double refraction 
is positive, in which it corresponds with diopside ; but the refractive 
power is very considerably greater in this black variety of augite 
