From A Text-book of Mineralogy by Edward S. Dana, 1905:

A highly serviceable microscope, for general use, is that described by Rosenbusch in 1876 and later much improved. A sectional view of one form is shown in Fig. 516, and a later and improved pattern is given in Fig. 517. The essential arrangements of Fig. 516 are as follows: The tube carrying the eyepiece and objective has a fine adjustment-screw at g the course adjustment is accomplished by the hand. The screw-head g is graduated and turns about a fixed index attached to the tube p; by this means the distance through which the tube is raised or lowered can be measured to .001 mm. The polarizing prism is placed below the stage at r, in a support with a graduated circle, so that the position of its vibration-plane can be fixed. The analyzing prism is contained in a cap, ss, which is placed over the eyepiece; this may be revolved at pleasure, its edge being graduated. When both prisms are set at the zero mark, their vibration-planes are crossed; when either is turned 90°, the planes are parallel (||). The stage is made to rotate about the vertical axis, but otherwise (in this simple form) is fixed ; its edge is graduated, so that the angle through which it is turned can be measured to 1/2 degree. Three adjustment-screws, of which one is shown at n, n, make it possible to bring the axis of the object-glass in coincidence with axis of rotation of the stage (see, further, the detailed drawing at the side).

The instrument here described may he used in the first place as an ordinary microscope with magnifying power adapted to the special case in hand. In the second place, with polarizing prisms and the usual arrangement of lenses, it serves for determining the planes of light-vibration (like the stauorscope of Art. 328); also for observing the interference-colors of doubly refracting sections and so on. Finally, with eyepiece removed and special condensing lenses added beneath the object on the stage (as more fully described later), it may be used, like the conoscope, for observing axial interference-figures, etc.

The microscope which has been briefly described is, as stated, especially applicable to tlie study of the form, optical properties, and mutual relations of minerals us they are in thin sections of rocks; it has therefore become an important adjunct to geological research. It can also be used to great advantage in the study of small independent crystals.