CHAPTER I. 
As noted in the introduction, the optical and crystallographical features, 
on which mineral diagnosis under the microscope is based, may be grouped 
into two classes: those of the first class (color, pleochroism and absorption, 
crystal habit, optical character of elongation, optical character of the mineral, 
dispersion of the optic axes, dispersion of the bisectrices] are ascertained ordi- 
narily by direct observation without measurement, while for the second 
class (refractive indices, birefringence, extinction angles, optic axial angles, 
cleavage angles) the numerical results of actual measurement are required. 
This distinction is drawn somewhat arbitrarily and is not meant to imply 
that the properties of the first group are strictly qualitative in their nature, 
but that they are treated at the present time in ordinary petrographic 
microscopic work as qualities of an object rather than quantities which 
must be definitely measured. With greater refinement in the methods of 
determination, some of these properties of the first class will undoubtedly 
be included in the second, essentially quantitative group. In this chapter 
the first or qualitative group of characters will be considered briefly and 
with special reference to their determination in fine-grained preparations. 
In the descriptions below, a working knowledge is assumed of the ordinary 
petrographic-microscopic methods, as treated in detail in the standard text- 
books on microscopical petrology. This assumption has been found neces- 
sary in order to save space and to avoid needless repetition of well-known 
methods. 
Microchemical methods will not be considered in the present paper. 
They are described in detail in the standard text-books, especially in the 
treatises by H. Behrens and by Klement & Renard. 
COLOR.* 
In the examination of thin sections of rocks one of the first features to 
attract the observer is the color of the different mineral components ; yet, 
until recently, there has been no satisfactory method for determining color. 
Radde's color scalef has been used for the purpose, but is not satisfactory 
for several reasons ; the comparison itself is exceedingly rough and the colors 
themselves, being largely of aniline or anthracene dyes, fade with time. 
Recently Mr. Frederic E. Ivesf placed the Ives standard colorimeter on 
the market and through his courtesy the writer has had opportunity to test 
and to use one of these instruments. Although the colorimeter in its present 
form is not intended primarily for use with the microscope, yet with the 
aid of a small prism both the field of the microscope and the colorimeter 
can be viewed side by side and the color of any mineral section matched 
in the colorimeter. The Ives colorimeter consists of three standard ray 
filters, red, green, and blue, so chosen that if the light from the three filters 
be observed simultaneously the effect is that of white light. In the con- 
*For a discussion of color scales, see P. G. Nutting, Bull. U. S. Bureau of Standards. 6, 89-93. 1909. 
tH. Fischer, Neues Jahrburh. 1879, 854-857; Rosenbusch-'W'uIn'ng. (I), I, 348, 1904. 
JThe Universal Colorimeter, The Ives Inventions, New York, N. Y. 
69 
