The President's Address, H. C. Sorhj, F.B.S., P.G.S. 13 
wliicli need be taken into consideration is a possible variation in the 
index of the balsam. I assume that it is the hard and brittle 
balsam used to fasten down the specimen before it is ground 
thin, and not the soft balsam used to fix on the covering glass. I 
find that the index of such hard balsam varies but little, and is 
about 1 • 54. 
As an illustration of what may thus be done, I will describe 
the results in the case of several different minerals in a section of 
a dolerite from near Glasgow, which is only about 4 Jo of an inch 
in thickness. I give the measurements in turns of the head of the 
fine adjustment. 
A colourless mineral containing fluid cavities, filling up cavities 
between the original minerals, was *207 turn in thickness, as 
measured in itself, and when compared with the hard Canada 
balsam the decrease in focal length in the latter was found to be 
• 907 _ • 007 
•007, whence we have / = 1-54 x = 1*49. In 
* Zi\j i 
accordance with the principles described in my address at the 
Mineralogical Society,* this clearly shows that this mineral is a 
zeolite, probably analcime. In a similar manner in the case of a 
mineral which looks very much like some variety of felspar, the 
focal length in the balsam was increased, and the index was found 
•214 + *025 
to be 1 • 54 X 7 ^]^ = 1*61, which clearly shows that it 
cannot be any felspar which contains a large amount of alkali, 
since that would reduce the index very considerably. Theory led 
me to conclude that the index of labradorite should correspond 
closely with this. I am not aware that its indices have been pre- 
viously determined. I found that they are about 1 • 621, 1 • 617, 
and 1^597. The mean of these is about 1^612, which agrees 
so closely with that of the mineral in the section, that it must 
almost certainly be labradorite, or some felspar of similar chemical 
composition. 
In like manner I found that the mean index and the focal 
character of the images given by another colourless mineral closely 
correspond with those characteristic of calcite. I also found that 
the mean index of a dark-coloured mineral was 1 ^79 or 1*80. No 
common silicate which does not contain much iron has so high an 
index. Both in this and in other optical characters it corresponds 
closely with the black augite in the lava of Vesuvius, which has a 
mean index of 1 * 785. 
The only important objection to this comparison with balsam is 
that its index may vary. It is, however, always possible to deter- 
mine what its real index is. Thus, for example, on comparing a 
* ‘ Mineralogical Magazine,’ vol. i. p. 193. 
