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. i represents 

 the natural size of a finished section of one of our diabase rocks. 



The i7ticroscope 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 Sections. X 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. 



f Jahrbtich fur Miner eralogie, Geologic, unci Palaeontologie , 1876, p. 504. 



X For a complete and systematic treatise on microscopic mineralogy, see Mikroskopische Physiographie der 

 petrographische ivichtigen Mineralien, von H. Rosenbusch. Stuttgart, 1873. See, also. Die Mikroskopische 

 Deschaffenheit der Mineralien und Cestine, von Dr. Ferdinand Zirkel. Leipsig, 1873. 



