21 



Hartnack's, and six oculars, two with micrometers, Calderon's 

 stauroscopic plates in one, Bertrand's quartz plate stauroscope in 

 another, and two with simple cross lines. There are supplied 

 also an objective micrometer, reading to * 1 mm. and additional 

 condensing lenses and diaphragms. 



59. Petrographic microscope. 



E. 68. 1888. Made by Seibert, Wetzlar. 



Usual horseshoe stand with tube rotating about a horizontal axis. 

 The stage is graduated and rotates. The centring is effected by 

 screws working on the stage, as in Fuess's model. The polariser 

 slides in, and is graduated. Over it is placed the condensing lens, 

 and diaphragms are provided to fit in the same place. The coarse 

 adjustment is by rack and pinion with one screw only ; the fine 

 adjustment, by a screw working vertically from the base, with 

 a graduated head. The analyser is placed in a slot, and does 

 not rotate, but is removable, and the gap in the tube closed by 

 a revolving shutter. Above the analyser is a rising and falling 

 tube within the main one, with a slot at the bottom to receive 

 a Bertrand lens, which is thus adjustable. Four eyepieces with 

 cross wires are provided, and a second analyser fitting over the 

 top with a graduated circle. There are five objectives, Nos. 1, 2, 

 4, 5, and 7, an additional Bertrand lens, an extra lens to fit 

 over the polariser, a box of selenite and quartz plates, a thick 

 plate of calcspar, and two test objects. 



60. Petrographical microscope. Price, 171. 



1887. Exhibited by J. Swift and Son. 



English tripod stand, with tube rotating about horizontal axis. 

 The stage is glass-covered and has a rim reading to 5'. Polariser 

 on a crank arm, rotating with slot for fixed position, graduated, 

 but not capable of vertical motion. A set of converging lenses 

 are provided to fit over it. Coarse adjustment by rack and 

 pinion, fine adjustment with graduated head. Optical tube 

 fitted with a nosepiece for the two objectives 1 in. and J in. pro- 

 vided. The main analyser fits into a slit near the base of the tube. 

 Below it is a slit for inserting a quartz plate. The eyepiece 

 has cross wires, and over it fits another analyser with graduated 

 rim, below which is an arrangement for inserting, rotating, and 

 removing a calcite plate. 



61. Petrographical microscope. 



1888. Exhibited by Watson, Bros. 



English tripod stand, with body movable about a horizontal axis, 

 and has an arrangement for fixing the tube in a horizontal position. 

 Has a large polariser on a crank arm with graduated edge with 

 lens, and extra condensing lenses fitting on above ; has a circular 

 stage, with rim divided to half degrees with vernier. Can be cen- 

 tred by use of a key ; superstage movable in one direction. Fine 



