126 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
cides with that for the upper nicol alone, but similar conclusions can be 
drawn as to the relative sensitiveness of the two methods, the one involving 
the rotation of the crystal plate (while the nicols remain crossed), and the 
second the rotation of the upper nicol while the crystal plate remains 
stationary. 
The amount of light required to produce the sensation of light in the 
human eye is different for different persons. But for a given eye the limit 
of the actual sensation of monochromatic light (threshold value) is fixed for 
any particular instant and may be represented by one of the horizontal 
percentage lines of the figures. Let us assume that for a source of mono- 
chromatic light of definite intensity /, the limit for the sensation of light is 
0.050 per cent of the total intensity and represented by the first horizontal 
line above the base line of Fig. 76. Then the curve for the crystal plate 
alone shows that for all points below that line, i. e., between 89 04' and 
90 56', the crystal will appear absolutely dark and on a single determination 
an error of nearly =*= i may be made. 
.20 
.10 
t 
90 
92* 
Fie. 78. In this particular case K is considered =o and the general formula reduces to 
/ i(i +cos 2<), which is independent of 9. In other words, if the thickness of the plate 
be such that sin'^d (/ a') o, or the emerging waves are any number of whole wave- 
lengths apart, total interference takes place and the plate is dark under crossed nicols 
for every angle of rotation about its normal axis. The curve indicates the change in 
intensity of field illumination on rotation of the upper nicol. 
If, however, the crystal plate remain stationary, and the upper nicol be 
rotated through small angles from its normal, crossed position (< = 88 to 
92), it is evident from the figure that if, for example, the crystal plate is 
30' distant from its position of total extinction and is still dark under crossed 
nicols so far as the eye of the observer can detect, the differences in intensity 
between the field and crystal plate for different angles of rotation of the 
upper nicol (measured by the ordinate intercepts between the curve o' and 
30' of figure) are of such a character that at the point where the illumination 
of the field can just be observed (88 43') the intensity of illumination of 
the crystal is more than twice as great (0.106 per cent instead of 0.05 per 
cent), whereas on the other side, where the first indications of illumination 
