THE PETROGRAPHIC MICROSCOPE. 
II 
ground, and this limit precludes the isolation and determination of the 
optical properties of a single grain, measuring much less than o.oi mm. in 
diameter, especially if it be birefracting. In aggregates of minute and over- 
lapping mineral grains or fibers, it is extremely difficult to single out one 
of the individuals and to determine its optical constants. The optical 
behavior of aggregates of minute superimposed mineral plates is often 
deceptive and may lead the observer to erroneous conclusions in regard to 
the optical properties of the substance under examination. In practical 
determinative work it is important, therefore, to make all determinations 
on separate grains which are clearly distinguishable and which alone are 
responsible for the optical phenomena observed. 
In view of the relatively low magnifications used in petrographic micro- 
scopic work, the strict attention to details of illumination and manipulation 
required in work with high powers is less essential to success in petrology 
than in biology or bacteriology, and for this 
reason the petrologist is often inclined to disre- 
gard them altogether and fails in a measure 
to utilize the possibilities of his instrument. 
In case, however, the microscope is to be used 
for determinative work with fine-grained prepa- 
rations, it is essential that these details be ob- 
served in a general way at least, and that the 
working parts of the microscope fit accurately 
and be in adjustment the accuracy of the re- 
sults being dependent in large measure on the 
accuracy of the construction and the adjust- 
ment of the instrument. For this reason it has 
seemed advisable to sketch in brief outline the 
general principles underlying the construction 
both of the optical and of the mechanical sys- 
tem of the microscope. 
The optical system of the ordinary microscope 
consists of four essential parts: 
(a) The reflecting mirror or reflector (Plate 
i, Fig. 4). 
(6) The substage condenser C. 
(c) The objective or object-glass 0. 
(d) The ocular or eye-piece . 
The petrographic microscope comprises in 
addition to these: 
(e) The lower nicol or polarizer P (Plate i , 
Fig. 4). 
(/) The upper nicol or analyzer A. 
(g) The Bertrand lens B. 
Each of these parts has a definite task to per- 
form and contributes its share to the efficiency 
of the whole. These parts are supported by a 
FIG. i. Section through 
microscope of Plate i, Fig. 3, 
showing working parts. The 
letters T, A, B,C,H,W,Q,F, 
O, U, V, E, L, and G refer to 
the same parts as in Plate i, 
Fig. 3 . 7 = lower iris d iaf ram ; 
N, upper nicol ; X, Y, Z, brass 
parts effecting rotation of up- 
per nicol; P, rotating stage 
supporting movable plate O. 
mechanical system, the important parts of which are (Plate i, Fig. 3) : 
(a) The stand or body for carrying the parts (6) (c) (d). 
