ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
239 
' Exner’s micro-refractometer (fig. 41) serves for the determination of 
the difference of refraction of two strongly refracting substances in a 
rock-section. The apparatus is brought over the end of the body- 
tube T in the position of the analyser. 
Fig. 42. 
An instrument suitable for instruction on the optical properties of 
rocks and minerals is the demonstration Microscope shown in fig. 42. 
The body-tube M slides in the socket H, and can be clamped by the 
ring K. The analysing nicol A is movable about a hinge, and can be 
rotated in its socket, which is provided with a division. Gypsum and 
mica plates can be inserted between it and the eye-piece. Above the 
