EXTINCTION ANGLES. 
133 
in such a way that the interference phenomena which it presents are mark- 
edly influenced by the slight disturbing effects from the crystal plate when 
it is not precisely in its position of complete extinction. 
SENSITIVE-TINT PLATE. 
In the discussion of the Newton color scale (page 108) attention was 
directed to the fact that when observations are made in white light, the 
interference colors which result on the insertion of a quartz or selenite plate 
in the diagonal position between crossed nicols resemble closely the Newton 
color scale. Under these conditions the intensity equation for waves emerg- 
ing from the quartz wedge reduces to /i = sin 2 - , which is identical 
A 
in form with that for the interference of white light reflected from two 
surfaces of glass or other material separated by a thin film of air. It will 
be recalled that the interference colors obtained from the wedge or crystal 
plate are dependent in a measure on the source of illumination used; also 
that the position and hue of the sensitive tint are not precisely the same 
for quartz, selenite, and air. A plate which shows the sensitive tint allows 
only about 6 per cent of the total light to be transmitted, with the result 
that such plates are low in visual intensity. 
The methods with the sensitive-tint plate are based on the fact that a 
slight change in path-difference in the transmitted light-waves produces a 
decided change in the interference color ; also that for certain colors the eye 
is very sensitive to a slight change in hue. The sensitive-tint plate is most 
effective on, and practically limited to, colorless plates showing low inter- 
ference colors of the first order. Its efficiency is seriously impaired in the 
case of deeply colored minerals which veil the true interference color and 
also with birefracting minerals thick enough to show interference colors 
higher than red of the first order. It can, moreover, only be used with 
white light and accordingly can not take cognizance of the dispersion of 
the bisectrices in the monoclinic and triclinic systems. Its color is, more- 
over, dependent to a certain degree on the 
source of light used. Methods with this 
plate are, therefore, not of general applica- 
tion and can be employed to advantage only 
under specially favorable conditions. In the 
case of exceedingly minute and weakly bire- 
fracting grains, it is often of advantage both 
in parallel and in convergent polarized light 
to rotate the sensitive-tint plate until its 
ellipsoidal axes practically coincide with the 
principal nicol planes. With this arrange- 
ment the field intensity is very low and the 
faint differences in color from the small 
mineral grain are more readily detected in 
the dull background than in the field flooded with light when the plate is 
inserted in the diagonal position. This applies with equal force to the 
double plates described in the paragraphs below. 
FIG. 79. 
