MICROSCOPICAL PETROGRAPHY 497 
has been found convenient. This ocular consists essentially of 
a metal holder, which fits into the microscope tube as an ordinary 
ocular and acts as a support for certain plates which are inserted 
in the lower focal plane of a Ramsden eyepiece above. Cross- 
hairs are attached to the base of the cylinder support for the 
Ramsden eyepiece and are viewed by it simultaneously with the 
upper surfaces of the inserted wedges or plates. 
For the measurement of the birefringence a graduated com- 
bination quartz wedge is used which is so cut that the 0.1 mm. 
divisions of the scale on its upper surface give directly path 
differences in 104 for sodium light. By its use together with a 
cap nicol, the path difference in sodium light or order of inter- 
ference color in white light can be readily ascertained, the wedge 
being inserted in the diagonal position until the black band of 
exact compensation is reached; the division of the scale covered by 
the black band is then the path difference in tov. In the case 
of thick plates the point of the line of compensation should be 
first determined approximately in white light; otherwise the 
correct line of compensation may not be selected when monochro- 
matic light is used. The path difference is directly dependent on 
two factors: the thickness of the plate and its birefringence. The 
simplest method for obtaining the thickness of a mounted plate 
or grain is to focus with a high-power objective first on its upper 
surface and then on its lower surface, as seen through the plate or 
grain itself. The amount of movement of the fine adjustment 
screw during this operation is the apparent thickness of the plate 
or grain, provided, of course, the fine adjustment screw is accurately 
constructed. The true thickness is obtained by multiplying the 
apparent thickness by the average refractive index of the plate. 
Experience has shown that under these conditions an error of 5 
or even Io per cent? may be made, especially if the plate be very 
thin, on thicker plates and grains the percentage error, due to 
imperfect focusing, is correspondingly less. To insure greater 
accuracy, the average of a series of determinations on the same 
plate should therefore be taken. 
tF. E. Wright, Amer. Jour. Sci. (4) XXIX, 415-26, 1910. 
2 See F. E. Wright, Amer. Jour. Sci. (4), XXIX, 416, 1910. 
