Joty—Identification of Orystals by the use of Birefringence. 489 
divergencies and corresponding colour changes. In other words, 
their effects are distributed over twice the range of colour-variation. 
The nepheline line exchanges its dull gray for white of the first 
order; the microline, anorthoclase, and orthoclase line enters on 
the yellow, sections assuming a straw yellow tint; the albite, 
oligoclase, and labradorite line rises into a distinct pale yellow ; 
the quartz-enstatite line rises to a bright yellow, while topaz and 
iolite enter the orange-yellow. Referring to the more strongly 
birefringent substances, we find anorthite rising from the palest 
_ straw yellow to a full orange-red; hypersthene will closely follow 
the same colour change. Tourmaline exhibits a conspicuous change 
of tint, from yellow of first order to green and bluish green of the 
second order. Augite rises from shades of orange-yellow of first 
order to corresponding shades of the second order, the colour- 
change being in amount just the one order. In a word, we find 
that a large number of the most important minerals—such as 
augite, tourmaline, hornblende, diallage, actinolite, which in the 
thickness of the normal rock-section show interference colours 
restricted to the yellows and oranges of the first order, in sections 
of doubled thickness—possess tints distributed over the much more 
varied hues of the second order, and with differences of retardation 
increased two-fold. Again, minerals which in normal rock-sections 
appear in the vivid colours of the second order become displaced 
into the pale and milky tints of the third and fourth orders. 
The optical arrangements whereby the polarized ray may be 
passed twice through the section are very simple. An“ opaque 
illuminator ” of the Zeiss, Leitz, or Nachet pattern may be used. 
Fig. 2 explains its construction. It will be seen that light 
entering through the opening in front of the prism is reflected down- 
wards through the objective and so on the object being examined, 
the ray being again returned by reflection from the object into the 
microscope and to the eye of the observer. In the figure the illumi- 
nator (which strictly consists of the collar and prism only, seen 
above the objective) is shown furnished with a nicol prism placed 
over the aperture. The illuminator thus reflects downwards on 
the object a polarized beam. ‘This addition is necessitated by the 
present method. The only other apparatus required is a small 
mirror to be placed immediately beneath the rock-section upon the 
stage of the microscope or laid on the upper surface of the Abbe 
