BIREFRINGENCE. 113 
In case a mineral plate appears colored its transmissivity varies through- 
out the spectrum; the more highly colored it is the greater the departure 
from uniformity in transmissivity. The interference color chart for such 
bodies is therefore totally different from the standard Newton chart; ab- 
normal interference colors result, as is evident from the blues and reds in 
chlorites and zoisite, from the yellows in epidote, etc. Such selective ab- 
sorption combined with the effects of differences in dispersion changes the 
series of interference colors from such minerals profoundly. 
Among the factors, then, which enter into the consideration of the inter- 
ference chart are standard white light of known intensity, the visual sensi- 
bility of the observer throughout the spectrum with varying intensity of 
illumination, the dispersion of the mineral plate for different colors, the 
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Wave lengths 
FIG. 71. In this figure the relative luminosity intensities of light throughout the 
visible spectrum are shown for plates of quartz (parallel and normal to the principal 
axis), of calcite and of selenite of such a thickness that the path-difference is 555 MM 
between crossed nicols for light of this wave-length. The quartz plate parallel to 
the axis shows the sensitive tint (red and blue ends of the spectrum about equal) at 
about 550 MM; the quartz plate normal to the axis, at about 547 MM; the calcite plate at 
about 551 MM, and the selenite plate at about 554 MM- 
selective absorption of the plate for different parts of the spectrum. To 
control all these factors at one and the same time and to ascertain the 
influence of each is not possible at present because of lack of precise data. 
In accurate determinations of birefringence it is accordingly advisable to 
use strong monochromatic light instead of white light. It is also evident 
that a revision of the Newton standard interference color scale with refer- 
ence to a standard white-light scale is necessary. The sensitive-tint plate 
and its relative accuracy will be considered further in Chapter IV on extinc- 
tion angles. 
G. C^saro* has suggested that compensation be referred to the sensitive 
tint at 28 1 MM between parallel nicols instead of absolute compensation. 
In view of the errors resulting from the white-light source, and from the 
dispersion and selective absorption of most mineral plates, this method can 
furnish only approximate results. 
In view of the different relative dispersions between minerals, it is evident 
that the interference colors from thick plates of a mineral can not be com- 
Bull. Acad. R. Belgique, a. Sci. (3). 26, 208. 1893. 
