Petrogra'phic Microscope. 



417 



dimensions be followed exactly, 1/10 millimeter divisions 

 ruled on the upper surface of the wedge (fig. 2b) will 

 give directly the difference in distance in fifju between emergent 

 light waves at a particular point. Thus, for sodium light the 

 distance between successive interference bands will be 5'89 mm . 

 The zero line of the scale must coincide precisely with the 

 black line of exact compensation between wedge and super- 

 imposed plate. In the present wedge this is the case ; the 

 slope of the wedge, however, is not exactly correct, and a 

 slight correction must be applied to the readings obtained 



Fig. 2a. 



Fig. 2a. Combination wedge. Upper part of combination quartz wedge 

 with, direction of elongations a, while lower part is a quartz plate, direction 

 of elongations c. 



since 22 mm on the wedge is equivalent to 22*62 /a/^. For inter- 

 ference colors of the first and second order this error (nearly 3 

 per cent) is very slight and can practically be neglected, but 



Fig. 2b. 



1 



O 50 100 150 ZOO ISO 50 



for higher orders it must be taken into 

 account and the readings multiplied 

 by a factor of proportion. In actual 

 work a table of equivalent values can 

 be prepared or an inclined line of 

 proper slope can be added to the color 

 chart and the correct value equiva- 

 lent to any observed value read off 

 directly. With proper care, however, 

 it is possible to grind these wedges 

 correctly, so that the 1/10 millimeter 

 divisions read directly in terms of dif 

 ferences in wave length (/a/a). 

 In practice, the determination of the birefringence of a 

 crystal plate in the thin section requires the determination of 

 two distinct factors — the thickness of the crystal plate and the 



Am. Jour. Sci.— Fourth Series, Vol. XXIX, No. 173.— Mat, 1910. 



28 



Fig. 2b. Top view of com- 

 bination wedge showing divi- 

 sions and position of dark 

 line of exact compensation of 

 wedge and plate. The 0*l mm 

 divisions on the scale are not 

 represented in the figure. 



