488 Scientific Proceedings, Royal Dublin Society. 
0:01, 0:02, &c., mms. Plotting in this manner the lines of the 
principal anisotropic transparent minerals, MM. Lévy and Lacroix 
have provided a chart from which by inspection the interference 
colour of crystal sections of known mineral species and thickness 
can be ascertained. 
Ordinary good rock-sections are about of the thickness 0-015 to 
0:02 mms. It follows that we find in such sections many of the 
most abundant of the rock-forming minerals showing but little 
differences in their maximum interference tints. Thus quartz, 
orthoclase, many of the plagioclase felspars, and even nepheline 
and topaz, may, so far as the colour diagnostic is concerned, be 
confounded in the more thinly cut sections. In order to evade 
this difficulty, a quartz plate may be used to raise the interference 
tint of the crystal and transfer it to the orange, reds, or blues, of 
the first or second orders, which then may be determined by a com- 
parateur. A glance at the chart (fig. 1) will, however, show that 
this cannot be expected to give distinctive results in all cases. We 
find, for example, that the sloping lines proper to quartz, plagioclase 
(for the most part), and orthoclase,-are so closely approximated by 
their convergence at the small thickness obtaining in good sections, 
that merely adding equally to the retardation produced by each 
substance, and thus shifting the whole series upward in the scale 
of colours, that is to the right on the chart, cannot produce dis- 
criminative differences. MM. Lévy and Lecroix recommend that 
the auxiliary quartz plate of known retardation be, for the sake 
of greater sensitiveness, cut to the thickness proper to the teinte 
sensible. It would appear, however, better in such cases to use a 
wedge of quartz, graduated on the edge, and in each case adjust 
it over the section till the tint of passage is found. 
A method which I have found of great use in increasing the 
discriminative value of birefringence can now be intelligibly 
described. It will at once appear that could we transmit the 
polarized ray ¢wice through the section under investigation an 
interference effect is obtained equivalent to that of a section of 
twice the thickness. We would, in such a case, without haying 
recourse to sections of a thickness unsuitable to observations with 
high powers, obtain the more distinctive interference effects of 
crystal plates 0°03 to 0°04 mms. thick. At this thickness the lines 
proper to the several substances have attained double their former 
