58 
Correspondence. 
be scientific study; but it would form a foundation for scientific study. It 
is even true that the well informed expert tends somewhat to lose sight of 
some of the principles involved in the phenomena which he witnesses, and 
by familiarity and long acquaintanceship with different minerals declare 
at sight that this is leucite, this is olivine, this is rhombic amphibole, and 
that is monoclinic pyroxene. It is not suggested that critical observations 
be undertaken; nor that research could be carried on without a sound in¬ 
sight into crystallographic and optical principles. The writer contemplates 
the acquisition of a stock of observed facts to which appeal may be made 
in subsequent illustration of theories. As our manuals are constructed, 
the student is first introduced to a study of explanations and then to a 
knowledge of the things explained. The natural process is inverted. 
In connection with the empirical course might well be associated the 
work of preparing thin sections. After practice in sections made in aim¬ 
less directions—more particularly in rocks—the pupil should have plain 
and detailed directions in the section-making of minerals in predetermined 
directions. By this time, if the pupil is not already acquainted with 
descriptive crystallography, he ought to acquire the rudiments of the sub¬ 
ject. He will thus learn, in due time, that the optical phenomena afforded 
by a species of mineral depend on the direction in which it is cut. Much 
profit would be gained if the manipulation did not proceed beyond sections 
of uniaxial minerals. These will illustrate most of the fundamental 
phenomena. 
The student now is in possession of a stock of facts, and he may justly 
be summoned to an exposition of underlying principles. With a recollec¬ 
tion of observed phenomena vividly in mind, the theoretical discussions 
will be vastly more intelligible than when they treat of a mass of indica¬ 
tions which have never been matters of observation by the pupil. In the 
study of optical principles it will place the learner at a disadvantage if he 
is put off with some abbreviated and generalized presentation. The mag¬ 
nitude of the science is no less when condensed into a few pages than 
wheD expanded into a general treatment. All therefore, which is not 
supplied by the book-help must be worried out by the thought and per¬ 
plexity of the learner. Difficulties greater than necessary drive the 
student toward abandonment of theory and repose in virtual empiricism. 
After familiarizing the learner with a wide range of phenomena, and 
after supplying him with ample theoretical explanation, the book help 
should afford detailed indications of the manipulations necessary in enter¬ 
ing on the investigation of an unknown mineral—say one detected in a 
crystalline rock. The analytical chemists have set the proper example in 
the formation of guidance-tables. Rosenbush has supplied a general table 
of the kind required. But each subdivision of it ought to be subjected to 
further analysis. In fact, the method might be carried to the actual de¬ 
termination of all the common mineral species. The tables of Hussak 
might here be made available. 
But the book-help might do more. It might well select for illustration 
some actual case of such character that the pupil could secure a duplicate 
of it, and carry the investigation through bj r an application of the analytical 
