Anniversary Address of the President, H.C. Sorby, F.R.S. 117 
Mounting Deposits in Balsam. 
In mounting the loose sandy deposits in Canada balsam I have 
found it a great advantage to adopt the following plan :—Having 
placed a very small quantity of dissolved gum on the glass plate, 
the requisite amount of the deposit is taken and mixed with the 
gum and sufficient water to make it easy to separate the grains 
and spread them uniformly over the space which will afterwards 
be covered by the thin glass. The water is then allowed to 
evaporate slowly, and though much of the gum collects round the 
margin, by properly regulating the quantity originally added 
enough remains under the larger grains to hold them so fast that 
they are not squeezed out with the excess of balsam. More gum 
than is sufficient for this purpose should not be used, since it may 
make itself too conspicuous in the object. When the proper quan- 
tity has been used, its presence can be detected only at the under 
surface of the grains, and in that situation does not in any way 
interfere with the study of the object. Independent of the conve- 
nience in mounting, this method prevents the grains from settling 
to one side of the object, even when soft balsam is used, which is 
desirable, since it penetrates more completely at a lower tempera- 
ture into irregularities of the surface and into the interior of 
compound grains than when harder. 
Visibility of the Objects. 
Some of the separate grains, or those enclosed in transparent 
minerals, do actually absorb a considerable part of the light trans- 
mitted through the object, and are therefore visible as black or 
coloured particles, quite independent of the angle of convergence 
of the light, and of the aperture of the condenser or object-glass. 
Many of the particles are, however, composed of quartz, mica, 
felspar, or other colourless and transparent ininerals, and when in 
water or balsam are made visible mainly by a portion of the light 
transmitted through them being so bent out of its course that it 
does not pass up to the eye-piece. ‘This is especially the case with 
the more or less curved edges of the grains, and with suitable 
illumination they thus show a dark outline. The same principles 
apply to fluid cavities in minerals, and to the accompanying bubbles. 
The width of the dark margin depends on the difference in the 
refractive power of the particle and of the surrounding medium, 
and if this difference be small, no such outline may be seen, if the 
angle of convergence of the light be at all considerable. Thus, if 
the aperture of both the object-glass and condenser is large, and 
the grains are mounted in Canada balsam, little or no trace of them 
may be visible, but by reducing the aperture their outline becomes 
more and more distinct, and the shading greater and greater, until 
