The Fresideyifs Address, H. C. Sorby, F.E.S., P.G.S. 17 
about T^io 0 of an inch, and the other by about 24 ^ 0 , the former 
being readily resolved by a f object-glass, but the other requiring 
a h. It will thus be seen that both classes of facts clearly prove 
that the mineral has two optic axes, and that the three indices of 
refraction differ very considerably, but have a mean value of about 
1‘71. This is *03 higher than that of the variety met with in 
the meteorites previously mentioned, probably owing to a variation 
in the amount of iron, though at the same time it is only fair to 
say that in measuring only once the indices in a section 3^0 of 
an inch in thickness, we cannot be sure that the second place of 
decimals is perfectly correct. On the whole, there can be very 
little doubt that it is olivine, since, so far as I am aware, no other 
common mineral has corresponding optical characters. 
In this section of lava are also scattered crystals of augite, their 
mean index of refraction being 1*80. By using a I could 
just separate one system of lines, but the section is too thin to 
enable me to separate the other system. In the case of another 
section, which is nearly three times as thick, I found that the 
indices were about 1 *80, I ’76, and 1*75, and I could just see that 
the second system was also double, but the lines much less separated 
than the others, which agrees with what would result from two of 
the indices being nearly equal. We thus prove most completely 
that the mineral has two optic axes inclined at a small angle, and 
has positive double refraction. In all these characters it differs so 
much from the olivine, that they could not possibly be confounded 
together, even if they were not otherwise well distinguished. I am 
not yet quite certain whether the above-described characters enable 
us to distinguish augite from hornblende. As far as my present 
observations go, the mean indices of refraction of the dark varieties 
of hornblende and of augite are nearly the same, but all my sec- 
tions of hornblende show the lines very decidedly less separated 
horizontally than in the case of augite of equal thickness, as though 
the double refractions were less intense. 
The principal difficulty in at once applying this method of 
study is that our knowledge of the indices of refraction of even 
some of the commonest minerals is so imperfect. Des Cloiseaux, 
in the ‘ Annuaire du Bureau des Longitudes ’ for 1877, gives what 
appears to be a very complete summary of what was then known. 
The minerals in this list are all but exclusively those which can be 
obtained in comparatively large and transparent crystals, since the 
indices could not be determined by the old method if they were small 
and somewhat opaque. In many cases also only the mean index 
of biaxial crystals is known, and not their three different indices. 
What we ought to know is the value of all the indices of the 
commoner and more opaque varieties of the constituent minerals of 
rocks. By the methods I have described these indices could soon 
VOL. I. c 
