18 



Biaxial Crystals. 



33. Adularia, S. Gothard. 



34. Anhydrite. 



35. Aragonite, Bilin. 



36. Axinite, Oisans. 



37. Barytes. 



38. Celestine, Erie. 



39. Diopside, Tyrol. 



40. Disthene, S. Gothard. 



41. Epidote, Tyrol. 



42. Mica. 



43. Gypsum, Sicily. 



44. Labrador! te. 



45. Cryolite, Greenland. 



46. Topaz, Brazil. 



47. Cordierite. 



C. Petrographic Microscopes. 



55. Petro graphic microscope. Designed by Prof essor 

 H. Bosenbusch, of Heidelberg., and described by him in 

 the Neues Jahrbuch fur Mineralogie, 1876. 



E. 88. 1885. Made by R. Fuess, Berlin. 



This is interesting as being one of the earliest microscopes 

 which were constructed for purely petrographical investigation. 

 It has the ordinary horseshoe stand of continental microscopes, 

 but is fixed in a vertical position. It has a sliding coarse ad- 

 justment, and reflector on a crank arm. It is provided with three 

 of Hartnack's objectives, Nos. 4, 7, 9, three eyepieces, and a lamp. 

 Its distinct petrographical features are as follows : -It has a 

 polariser easily thrown in and out of position, and the analyzer 

 which fits over the eyepiece has a graduated ring, whose zero 

 corresponds to the crossing of the Nicols. The stage rotates, and 

 is graduated in degrees on the circumference, and along two 

 lines at right angles on the surface in millimetres, for the pur- 

 pose of recording the position of any spot in the slide examined. 

 The accurate centring of the ocular portion, so that its axis may 

 pass through the centre of rotation of the stage, is secured by 

 means of a second tube outside the first which supports the inner 

 by springs, and contains two screws, which work near the end 

 of the tube carrying the objective, so as to move the latter in 

 either or both of two horizontal directions at right angles to each 

 other. Two eyepieces are provided with cross lines. The 

 fine adjustment has a graduated circle. There is a slit in the 

 side immediately above the objective, for the insertion of a 

 quartz plate or wedge, and there is also a calcite plate to fit 

 over the eyepiece. An apparatus for heating sections during 

 examination consists of a long hollow metal box, at one end of 

 which is placed a lamp within a mica shade, by means of which 

 the air within the box is heated, and communicates its heat to 

 the slide, which is placed over a perforated disc in the centre, 

 the perforations being covered by glass and a thermometer with 

 a branched bulb, placed with one branch on each side of the 

 object. To this instrument a Bertrand-Lasaulx combination 



