8 MINERALOGY AND LITHOLOGY. 
the warm balsam on the clean side; then, by pressing it down and working it back 
and forth with the knife-point, the specimen can be got into good position, and all the 
bubbles of air can be removed from between the two glasses. When the glass is cold, 
the superfluous balsam is removed as far as possible with a heated knife-blade, and the 
balsam that remains is washed away with alcohol, which dissolves it; and the section, 
on being wiped dry with a clean cloth, is ready for examination.* Fig. 1 represents 
the natural size of a finished section of one of our diabase rocks. 
The microscope that has been used in this work is one which was described by Rosen- 
busch.f The essential features of a microscope for the examination of mineral sec- 
tions, beyond -those required for ordinary microscopic work, are,—Nicol prisms, so 
arranged and attached to the instrument that the plane of vibration of the light which 
passes through them can be determined, and some arrangement by which a section of 
a mineral can be brought into position in the field of the microscope in any desired 
relationship to these planes. In the microscope mentioned, this is effected by con- 
structing a graduated circle on the mounting of each Nicol, and by placing a hair cross 
in the ocular. The tube and ocular of the instrument are not revolvable; and the 
adjustment for focusing is a vertical motion of one tube that slides up and down 
within another, but does not revolve within it; and if the zero of the graduated circle 
on the Nicol below the stage and the zero of the other that is placed over the ocular 
are both placed at the lines drawn upon the instrument for that purpose, the Nicols are 
crossed, and the two arms of the hair cross in the ocular correspond to the planes of 
vibration of the Nicol prisms. If now any section is brought into the field of the 
microscope, the relationship which the edges or cleavage lines of a crystal bear to the 
plane of vibration of the light which illumines it is shown by their relationship to the 
fixed hair lines in the ocular. In the excellent, inexpensive instrument that I have 
mentioned, a number of beautiful devices are introduced for the sake of making accu- 
rate work possible. A basal section of clacite can be placed on the ocular under the 
upper Nicol, or a quartz plate can be placed in the tube directly over the objective, 
either of which arrangements makes an excellent stauroscope of the instrument. To 
the applications of these, reference will presently be made. I have now mentioned 
the really essential arrangements that can, with a little trouble, be placed upon any 
microscope. 
Examination of Sectzons.{ The first point that will be noticed on examining a sec- 
tion of a mineral is its purity or impurity; and very often a mineral that is apparently 
# Those who enter extensively into the study of rocks usually use somewhat more expensive apparatus. A 
lathe which rotates a plate for grinding, and a disc for sawing, are commonly employed. It may also be men- 
tioned that some persons make a business of preparing such sections, and, with their experience, can make sec- 
tions in any given direction, through crystals or rocks, and can fulfil any specifications that may be made. Mr, 
L. Stadmuller, of New Haven, is one who prepares such sections; and Mr. Alexis Julien, of the Columbia Col- 
lege School of Mines, makes most satisfactory preparations. 
+ Yahrbuch fur Minereralogie, Geologie, und Palaeontologie, 1876, p. 504. 
{For a complete and systematic treatise on microscopic mineralogy, see Mikroskopische Physiographie der 
petrographische wichtigen Mineralien, von H. Rosenbusch. Stuttgart, 1873. See, also, Die Mikroskopische 
Beschaffenheit der Mineralien und Gestine, von Dr. Ferdinand Zirkel. Leipsig, 1873. 
