F. E. Wright— Measurement of Extinction Angles. 369 



darkness is attained. This method is in general nse and is 

 equally well adapted for white light and for monochromatic 

 light. With it any degree of accuracy can be attained provided 

 a sufficient number of measurements be taken to reduce the 

 probable error. In applying this method it is customary to 

 note not only the positions of maximum darkness attained by 

 the crystal when revolved clockwise from a position of bright 

 illumination, but also when revolved counter clockwise from 

 such position. This was the method used by Max Schuster* 

 in his classic measurements of the extinction angles of plagio- 

 clase feldspars. He determined for each cleavage flake the 

 position of zero extinction eighty times for clockwise revolu- 

 tions of the plate and eighty times for counter clockwise 

 revolutions, and averaged the two readings. His work in this 

 line remains unsurpassed, even to the present time. 



To increase the accuracy of each determination on a crystal 

 plate under crossed nicols, different schemes have been devised, 

 all of which depend on the disturbing influence of the plate on 

 inserted plates or wedges of birefracting substances. Each of 

 the inserted plates or wedges is constructed in such a way that 

 the interference phenomena which it presents are markedly 

 influenced by the slight disturbing effects from the crystal plate 

 when it is not precisely in its position of zero extinction. 



Sensitive Tint Elate. — Plates showing this interference 

 color (violet of the second order) are usually made of selenite 

 or quartz and are under certain conditions very sensitive to the 

 slight changes which the crystal plate produces when it is not 

 precisely in its position of total extinction. As a general rule, 

 the eye is more sensitive to slight differences in color than in 

 intensity, and in certain cases the sensitive tint plate can be 

 used to advantage to increase the accuracy of the ordinary 

 method. Its use is most effective on and practically limited to 

 colorless plates showing low interference colors of the first 

 order. Its efficiency is seriously impaired in the case of deeply 

 colored minerals which veil the true interference color and also 

 in thick plates of strongly birefracting minerals showing high 

 interference colors, even red of the first order. It can, moreover, 

 only be used with white light and accordingly cannot take 

 cognizance of the dispersion of the bisectrices in the mono- 

 clinic and triclinic systems. This method is therefore not of 

 general application and can be employed to advantage only 

 under specially favorable conditions. 



Bravais\-Stoher% plate. — This plate is also cut to show the 

 sensitive violet interference color, and consists of two such 



*Tschermak'sMm. Petr. Mitth. v, 189, 1882. 



■f Comptes Kendus, xxxii, 113, 1851 ; also Pogg. Ann. xcvi, 39T, 1855. 



X Zeitschr. Kryst. xxix, 22-24, 1898. 



