100 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
it is evident that the principal planes are inclined to the plane of projection ; 
also that if A\ and A* be the optic axes there is a direction d for which the 
plane of vibration is in the plane of incidence, as required both by theory 
and observation. 
In observations with the total refractometer, it is important that the 
instrument be in accurate adjustment.* Unfortunately the crystal re- 
fractometers now on the market are not designed with reference to adjust- 
ment facilities and their adjustment is often an exceedingly difficult and 
tedious matter. In observing the boundary curves from small mineral 
plates it is essential that the source of light be imaged in the object plane, 
and again together with the object in the eye-circle of the ocular of the 
telescope. In this eye-circle the light from all parts of the field except the 
mineral plate can be cut off by means of a proper stop or diafram and the 
boundary curves from plates i mm. in diameter readily observed. In the 
case of a finely crystalline aggregate the writer has been able, in certain 
instances, to observe the limits on the boundary curves which appear under 
these conditions as indistinct shadows and which correspond to the greatest 
and least refractive indices. For practical purposes, it is convenient to 
prepare a curve plotted on a large scale on which the refractive index cor- 
responding to any observed angle of total reflection can be read off directly 
to the fourth decimal place. With accurate work, the probable error of 
refractive index determinations by this method on suitable plates is not over 
2 in the fourth decimal place. The interference fringes which appear occa- 
sionally when the observations are made in reflected light can usually be 
decreased by using a higher refractive liquid or by increasing the distance 
between the plate and the glass hemisphere by means of some small support, 
as a paper or cork ring. 
The total refractometer of WalleYant, which is attached to the micro- 
scope and is employed only on thin uncovered and polished sections of 
rocks, has proved a useful instrument in the hands of Professor WalleVant, 
who has applied it to the determination of the feldspars and other minerals. 
The present writer has unfortunately had practically no opportunity to 
work with this instrument and is not in a position to judge of its fitness for 
application to fine-grained preparations. Obviously it can not be used on 
preparations whose granularity is expressed in hundredths of millimeters. 
Compare P. E. Wright: Die Justierung des Abbe-Pulfrichschen Kristallrefraktomctcrs, Zeitsch. f. 
Instrumentenkunde, 28, 201-206, 1908. 
