700 
the subject in their published “ Proceedings.” But the fluxion -structure is the most 
striking, and it sufliees to illustrate, in this particular direction, the value of the 
microscopical examination of rocks. 
The microscopical examination of its section does more, however, than merely 
disclose the structure of the rock, for, by using polarized light instead of ordinary light 
and an analysing Nicol’s prism (usually referred to as a “Nicol”), the observer goes 
further, and is able to identify the different component minerals constituting the rock. 
It is hardly possible to explain, in a short introductory chapter, how this is 
done ; but, to simplify the matter so that general readers may be attracted to, and not 
repelled from, what is really a most attractive subject, 1 will try to explain how 
polarized light differs from ordinary light. 
If we imagine a ray of ordinary light to be magnified many millions of times, 
then that portion of a test-tube cleaner which carries bristles, or the same portion of the 
cleaner sup])lied with an infant’s feeding bottle, may represent, roughly, the ray of 
ordinary light, the wire being the path, and the bristles representing the cross vibra- 
tions of the ether- particles. By arranging the bristles parallel to each other a fair 
illustration is obtained of the polarized ray. 
Thus, then, in ordinari/ light, the ether-particles vibrate in all directions across 
the path of the ray ; and in polarized light, the ether particles vibrate in parallel planes 
across the j)ath of the ray. 
Now, to produce polarized light we can compel the ether-particles to vibrate in 
parallel planes, in two ways — either by rejleclion from glass or polished metallic surfaces, 
at certain angles, dependent on the nature of the reflecting material ; or by refraction 
through crystal plates. N ieol (a Scotch professor) found out how to obtain this polarized 
light by compelling it to pass through prisms of Iceland spar (a variety of calcite), 
specially cut and jirepared; and this is the method universally adopted in petrography 
at present. 
To recognise when light is polarized requires a similar prism or reflecting plane. 
Either docs, no matter what means are employed for polarizing the light to be examined. 
In the microscopes used for petrographical work there is a polarizing prism 
beneath the stage, and a precisely similar prism over the eye piece. Both can be revolved ; 
and by adjusting the analysing prism so that its principal axis is at right angles to that 
of the lower polarizing prism, perfect darkness covers the field of the microscope, and 
the nicols are technically known as “ crossed.” 
By turning the analysing prism the field gradually gets illuminated till at 90° 
the maximum of illumination is reached, and the nicols are said to be “ parallel.” A 
continued revolving of the analysing prism would lead to the gradual darkening of the 
stage until no light passed through, when the nicols would be again “ crossed.” 
Most of the drawings illustrating these Notes represent sections as viewed between 
crossed nicols, and under such circumstances the different minerals exhibit characteristic 
optical properties. For instance, the bands appearing in one of the minerals in the drawing 
of the Cape Upstart section certainly prove it to be one of the felspar group of minerals. 
Again, colour is of some value, certain minerals being illuminated vividly, like the augite 
crystal depicted on Plate G5, fig. 2 ; while orthoclase is always a delicate lavender grey. 
These colours vary, however, being dependent on the thickness of the section. 
There is one system of crystals belonging to the cubic form and its derivatives, 
none of which are illuminated at all, even in thick sections.* To this group belong 
fluorspar, garnets, &c. They are, therefore, easily recognised. 
» A thin section may be about '001 inch or less, while a thick section would be about '006 inch or 
even thicker. 
