14 SORBY : OPTICAL CHARACTERS OF MINERALS. 
section of dolerite from near Glasgow, which, on an average, 
is about ^o-th of an inch thick. 
I found that the index of a colourless transparent mineral, 
filling up cavities between the original minerals, was about 
1*48 or 149. This exactly corresponds with that of analcime, 
with which its other optical characters agree. 
Another colourless mineral, also filling cavities, was found 
to have the indices and other characters of calcite. 
A third colourless mineral, evidently an original con- 
stituent, was seen to have a comparatively feeble double 
refraction, and its index was found to be 1'61. Its general 
appearance was like that of some felspar, but this index 
clearly proves that it cannot be any species which contains 
a considerable amount of alkali, which would greatly reduce 
the refractive power. The index of labrodorite was not 
previously known, but I find that it is 1 "61, and therefore 
there can be little doubt that the mineral in the section is 
that species. 
The section also contains a number of transparent reddish- 
brown crystals, their index of refraction being about 179. 
This and their other optical characters closely agree with 
those of the dark augite in the lava of Yesuvius. 
In now concluding this short address, I cannot but feel 
that I have been obliged to omit all allusion to many points 
of considerable practical importance. I have not attempted 
to describe the subject in such a manner as would enable any 
one to at once practically apply the method in all sorts of cases. 
I gave a somewhat full account of one branch of the subject 
in my address at the meeting of the Mineralogical Society 
at Plymouth, and entered into the more purely microscopical 
aspect of the question in my late address at the anniversary 
meeting of the Royal Microscopical Society. I propose to 
communicate a detailed paper to the Eoyal Society as soon 
as a correct explanation can be discovered of certain small 
