23 



rotating stage, graduated to degrees. The polariser is on a 

 crank arm, and has spring catches to fix its diameter at right 

 angles to that of the analyser. The centring is secured by 

 screws working at right angles on the objective holder. The 

 eyepiece has cross lines, and is fixed by means of a slot. The 

 analyser, which is graduated, fits over it and can be rotated, 

 and has spring catches to indicate the crossing with the polariser. 

 It has rackwork coarse adjustment, and micrometer screw fine 

 adjustment, not graduated. It is supplied with 1 in. and in. 

 objectives. This instrument can also be provided with a set of 

 converging lenses to produce the interference figures, and is 

 sometimes made with a slit for the insertion of a quartz-plate 

 and quarter-undulation-plate above the objective. 



66. Petrographic microscope with inclinable tube on 

 bronzed tripod stand. Price SI. Ss. 



1889. Exhibited by H. Crouch $ Go. 



The polariser is carried by a crank arm ; the analyser is- 

 contained in a block, which slides in a slot above the objective. 



The stage rotates, and is graduated. 



One eyepiece is supplied, which is fitted with an indicator ;. 

 and a revolving double nose-piece carries a J in. and 1^ in. 

 objectives. 



E. Apparatus for cutting Sections of Minerals and 



Rocks. 



67. Lapidary's wheel for cutting large specimens. 

 E. 39. 1884. Made by R. Fuess, Berlin. 



This is adapted for cutting sections several square inches in 

 area. It has a heavy cast-iron stand and plain wood treadle. 

 The axle of the wheel is attached to one side only of the stand ; the 

 diameter of the wheel is about 22 in. The upper part of the casting 

 consists of two broad parallel bars, leaving a groove between, 

 which is edged with steel, and along which the movable parts 

 can slide and can be fixed by nuts below. The first of these 

 movable parts consists of the small wheel, with a diameter of 

 6 in. on a double horizontal support, the other end of its axle 

 carrying the vertical slitting disc, round which, except near the 

 base is a loose tin case. The second movable piece is the rock 

 carrier. This consists of a platform fixed on the short arm of a 

 lever, whose fulcrum rests on the slider, and to which it can be 

 secured by nuts. It can thus be fixed in any required horizontal 

 position, and it can also be raised or lowered for changing the 

 vertical position. The other end of the lever is a long bar 

 carrying cylindrical sliding weights, by which the upward 

 pressure of the platform end can be modified at pleasure. On 



