i88o.] 
Analyses of Books . 
67 
matter, and those employed for demonstrating the structure of 
formed material ; the dyes differ widely both chemically and in 
the mode of their application. , 
3. The examination of various substances, both organic and 
inorganic, is in this part treated still more m detail. The pro- 
cesses used in the examination of rocks and fossils have been 
contributed by H. C. Sorby, F.R.S., and Frank Rutley, of the 
Geological Survey. Mr. Sorby’s remarks on the use of the 
Polariscope are worth quoting at length, as the value of this 
appendage to the microscope is still far from being generally 
understood. Mr. Sorby writes : — ' “ Polarised light must not be 
used simply to show structure, or, as is too often the case, 
merely to show pretty colours, for it is a most searching means 
of learning the nature and molecular constitutions of the sub- 
stances under examination. The attion of crystals on polarised 
lio-ht, as applied to the microscope, is due to their double retrac- 
tion, which depolarises the polarised beam, and gives rise to 
colours by interference, if the crystal be not too thick m propor- 
tion to the intensity of the power of double refradhon in the line 
of vision. This varies much, according to the position in which 
the crystal is cut, and therefore, in a sedhon of a rock, dlffe rent 
crystals of the same mineral may give very different results ; but 
still we may often form a good general opinion on the intensity, 
and may thus distinguish different minerals whose intensity of 
adtion varies considerably. But besides this, the intensity, but 
not the charaaer, of the depolarised light, varies according to 
the position of the crystal in relation to the plane of polarisation 
of the light. There are two axes at right angles to each other, 
and when either of them is parallel to the plane of polarisation 
the crvstal has no depolarising adtion, and if the polarising and 
analysing prism are crossed the field looks black. On rotating 
either the crystal or the plane of polarisation the intensity of de- 
polarising adhion gradually increases, until the axes are inclined 
to 4<°, and then gradually diminishes till the other axis is in the 
plane of polarisation. If, therefore, we are examining any 
transparent body, and find that this takes place uniformly over 
the whole, we know that the whole has one simple crystalline 
struaure : whereas if it appears as it were to break up into de- 
tached parts, each of which changes independently, we know 
that it is made up of a number of separate crystalline portions, 
either related as twins or quite independent of each other, as 
other faas may indicate. By using a plate of selenite of suit- 
able thickness, we may also ascertain in what directions the 
crystal raises and depresses the tint of colour given by the 
selenite, and can thus determine the position of the principal 
axis of the crystal.” . . 
4. Treats of Chemical Analysis applied to microscopical inves- 
titrations. The portion devoted to Spectrum Analysis is from 
the pen of H. C. Sorby, F.R.S., and scarcely needs comment. 
A list of works on the subjedt is appended. 
