PETROGRAPHIC MICROSCOPE AS ACCURATE MEASURING DEVICE. 57 
THE PETROGRAPHIC MICROSCOPE AS AN ACCURATE MEASURING 
DEVICE. 
In the preceding pages attention has been directed to the importance of 
the optical system of the microscope and to some of the details which the 
optician has to observe in designing and constructing his system. The 
second function of the petrographic microscope, that of an optical meas- 
uring device, introduces a number of new factors into its construction 
which have to be considered and which vary to some extent with the pur- 
pose which the microscope is to serve. The importance of accurate con- 
struction and of adjustment is obvious. In quantitative work it is essential 
that all instrumental errors be reduced to a minimum and that their effect 
on the final result be definitely known. The demand for accurate numerical 
data in modern petrography is increasing and is the natural outcome of 
the growth of the science. Notwithstanding this tendency the quantitative 
element is still absent in many petrographic descriptions and the reason 
for it is evidently to be sought in the cumbersome methods and appliances 
now available for the determination of the optical constants of minerals 
under the microscope. The observer has not the time to carry out such 
measurements properly and in consequence does not attempt them at all. 
If, however, the appliances could be simplified so that the different meas- 
urements could be made easily and rapidly and the correction factors 
eradicated these objections would be removed and petrography might 
profit accordingly. 
In the microscope pictured in Plate 2, Fig. i, the attempt has been made 
to devise an instrument which is simple and yet with which most of the 
optical constants can be determined directly and accurately without the 
use of complicated accessory apparatus. The microscope of Plate i,Fig. 3, 
is intended for precise work and contains several features which have not 
been included in that of Plate 2, Fig. i, and this in turn has some features 
not included in Plate i, Fig. 3. Several of these features are in part new 
and merit a brief word of description. 
(i) For the simultaneous rotation of the nicols a rigid-bar connection 
has been adopted, which is free from lost motion and the inaccuracies of 
gear-wheel devices which are ordinarily used for this purpose. The rigid- 
bar connection between the two nicols was first used by Mr. Allan B. Dick* 
in 1888. In his arrangement the connection was made between the polar- 
izer and a cap nicol above the ocular and was intended as an inexpensive 
substitute for the pin and ratchet movement with gear wheels, also first 
devised by him. The connecting bar was broken at five points, was long, 
and could hardly furnish results of a high order of accuracy, and was evi- 
dently not intended to do so, as the small degree circles without verniers 
testify. 
In 1904 E. Sommerfeldt described a microscope especially adapted for 
work with high temperatures; in it the two nicols were connected by a rigid 
bar as in the Dick microscope; the angle of rotation of the polarizer and 
cap nicol were read off directly on the stage of the microscope. In 1905 
*Miner. Mag., 8, 660, 1888: Notes on a new form of polarizing microscope, 1890. Some additional notes 
on the petrographic microscope. Published by James Swift and Son, London. 1804. 
