8 
POPULAR SCIENCE REVIEW. 
is rotated a few degrees, and the prisms again crossed by 
moving the analyser, the felspar transmits light.” Thus it is 
shown that a felsitic base, that at the first glance may appear 
glassy, will exhibit double refraction. When polarised it will 
break up as the prisms are rotated into variously-coloured little 
patches, which gradually assume a more definite form as the 
axes approach a right angle. In that position it appears as a 
granular compound of crystalline fragments, amongst which are 
a few more or less perfect crystals. It has evidently been con- 
solidated whilst undergoing crystallisation. We meet with this 
kind of base in felsites, porphyrites, &c. The quartz of granite 
might also at first glance be mistaken frequently for a glassy 
base, as it is found without any crystalline form, enveloping all 
the other minerals. It is seen to be structureless under the 
microscope, and containing numerous fluid and other cavities. 
But directly it is examined with polarised light, the rotation of 
the analyser produces a gorgeous display of colours, broken into 
irregular patches, refracting different tints. Some of the 
patches show round their edges parallel wavy bands of colour, 
marking out the individuality of the patches, and showing the 
manner in which its constituent particles consolidated in inde- 
pendent masses.* 
I will now endeavour to point out some of the chief dis- 
tinguishing marks by which we may recognise the microscopic 
forms of the minerals ordinarily met with in these igneous 
rocks. The rock to be examined may be either a granite or a 
dolerite of the carboniferous period, or a more recent trachyte, 
or a modern lava, and such a rock may contain as its most fre- 
quent constituents three or four or more of the following mine- 
rals : quartz, felspar of various species, mica, hornblende, 
augite, epidote, olivine, leucite, nepheline, apatite, chlorite, 
nosean, schorl, calcite, as well as some others not quite so 
common. We must, of course, have our specimen which is to- 
be examined cut sufficiently thin to be transparent, and we 
must study it both with ordinary and with polarised light. 
We shall see that some minerals present appearances which are 
perfectly recognisable in ordinary light, but for the distinction 
of the greater number of microscopical minerals the use of the 
polariscope cannot well be dispensed with. And here a word 
may be said as to the practical use of this instrument. It will 
enable us readily to distinguish between single and double re- 
fracting substances ; minerals, for instance, of the cubic from 
those of other systems. All who have made any use of the 
polariscope know that when the prisms are crossed the field be- 
comes dark, the polarised beam being unable to pass. Now, if 
S. Allport, F.G.S. 
