INTRODUCTION. 9 
reason it is necessary, in judging of a method or of a piece of apparatus, to 
note its general applicability and its accuracy under the different conditions 
which may arise. A device which is so constructed that its sensibility is 
variable and may be changed to meet the different possible conditions is 
obviously better than one which gives satisfactory results for only a fixed 
set of conditions. The question of adjustable sensibility is especially impor- 
tant in methods based on differences in light intensity or on the interference 
phenomena of white light; and several of the devices described below (the 
double combination wedge, the bi-quartz wedge plate, the artificially 
twinned quartz wedge) have been constructed with this special end in view. 
In the preparation of the descriptions, constant use has been made of the 
standard works on microscopical petrography, especially of the Mikro- 
skopische Physiographic d. Mineralien, ed. 4, by Rosenbusch-Wulfing; of 
the Trait^ de Technique MineYalogique et Petrographique, i, by Duparc- 
Pearce; and of Rock Minerals by J. P. Iddings. For the sake of brevity, it 
has been found expedient, in not a few of the methods noted below, to 
indicate merely their salient features and to refer to the standard works or 
the original articles for their more complete description and elucidation. 
My thanks are due to Mr. E. S. Larsen, formerly of the Geophysical 
Laboratory, and to Dr. H. E. Merwin and Dr. C. N. Fenner, of the Geo- 
physical Laboratory, who have aided me from time to time in testing the 
different methods described below. I am also indebted to Dr. P. G. Nutting 
for advice in several optical matters; and especially am I under obligations 
to Dr. H. Kellner, of the Bausch and Lomb Optical Company, Rochester, 
N. Y., who has read the introduction and has made many valuable sugges- 
tions in regard to the presentation of the optics of the lens system of the 
microscope. 
