12 



41. Contact goniometer. 



E. 87. 1885. Made by R. Fuess, Berlin. 



Consists of a broad steel arc graduated from to 90, the 

 same subdivisions being also marked from 90 to 180 for con- 

 venience ; radially to this are two bars with straight edges over- 

 lapping in the centre, the one is fixed at and the other is mov- 

 able by means of a carrier fitting over the arc, the straight edge of 

 which passes over the subdivisi<5ns. Both arms slip in a radial 

 direction in their sockets. 



42. Contact goniometer. 



E. 86. 1885. Made by R. Fuess, Berlin. 



A silvered brass circular flat ring, graduated to degrees. Flat 

 radial bars slide in slots in the direction of their length, and 

 can then be fixed so as to move only about the centre. The index 

 and arms are of steel, and both arms are movable. 



B. Instruments for investigating the Optical Properties of 



Minerals. 



43. Haidinger's dichroiscope. 



E. 90. 1885. Made by R. Fuess, Berlin. 



At one end of the tube is a convex lens to which the eye is 

 applied ; at the other, a small square hole ; between them is 

 a rhombohedron of Iceland spar, so cut that the two surfaces 

 are opposite rhomb faces, the shorter diameters of which are 

 parallel to the median plane of the tube. The thickness is so 

 chosen that the two images of the square aperture seen through 

 the lens at the other end may lie side by side. When a plate of 

 crystal in which the ordinary and extraordinary rays are separated 

 is placed before the aperture, the one image formed by the di- 

 chroiscope when its axial plane is parallel to that of the crystal, 

 will be formed by the ordinary and the other by the extraordi- 

 nary ray. If there is any difference in colour or intensity 

 between these, as they are side by side it may be immediately 

 perceived. 



44. Brezina's stauroscope. 



E. 89. 1885. Made by R. Fuess, Berlin. 



An apparatus for determining the positions of the planes of 

 vibration in crystal sections, by their coincidence or otherwise 

 with the planes of vibration in the polarizer or analyzer. The 

 instrument is vertical and consists of a tube carrying lenses 

 above, and another tube carrying lenses for illuminating the object 

 from below. The stage rotates and is graduated, and there is a con- 

 Cave reflector on the stand. The polarizing portion consists of a 



