486 FRED, EUGENE WRIGHT 
from the laboratory, been felt in petrology as it is today. The 
pioneers have done the reconnaissance work for us and we must 
proceed along quantitative lines of attack before we can hope to 
obtain even approximate solutions of the big problems ahead. 
Each one of us may contribute his share and add his little stone to 
the structure by adopting the quantitative viewpoint and realizing 
its importance in his attitude toward science in general and 
petrology in particular. In petrology the quality of our quantitative 
work is far more important than the quantity of our qualitative work. 
Having now considered the standards postulated by modern 
petrology, our next step will be to show that the present-day tools 
and methods of microscopical petrography measure up to these re- 
quirements and are furthermore easy of application and simple in 
principle. We base our judgment of the value of a tool or method 
on its effectiveness, its simplicity, its adjustable sensibility, and 
its range of applicability. An instrument whose sensibility can be 
adjusted to meet the different conditions of observation which may 
arise is obviously superior to an instrument whose sensibility is 
rigidly fixed and adapted for only one particular set of conditions. 
The range of applicability as a feature in any instrument should 
not be carried too far because practical experience has shown clearly 
that the so-called universal instruments are, as a rule, unsatisfac- 
tory and often do not accomplish in a thoroughly competent 
manner any one of the several purposes for which they are intended. 
To fulfil a given set of conditions adequately, it is usually neces- 
sary that a special instrument be designed for the purpose. Thus, 
a small caliber rifle may be admirably suited for small game, but 
for larger game it is wholly inadequate, and may do more harm 
than service in an emergency; vice versa, a large caliber rifle is of 
little value for hunting small game. 
The first tools which are devised for a given purpose are usually 
affected by “‘children’s diseases,’ as Dr. A. L. Day has expressed 
it, and only by careful mechanical attention can such troubles be 
eliminated. The instruments and methods of microscopical 
petrography have in large measure passed through and beyond 
this stage and have been developed to such an extent that prac- 
tically all the optical properties of mineral grains can now be 
