22 



adjustment in front ; coarse adjustment by rack and pinion. 

 A slot above the objective for the insertion of quartz-plate, 

 quarter-undulation-plate, and Bertrand-lens, which are pro- 

 vided. The analyser fits over the eyepiece, which is of low 

 power and provided with a graduated circle, and an additional 

 eyepiece is provided. There are three objectives of 1J in., | in., 

 and in. focus. The accessories are a condenser on stand, a 

 live box, stage and hand forceps. 



D. Petrographic Microscopes for Students. 



62. Petrographic microscope. 



E. 116. 1886. Made by A. Nachet, Paris. 



As in the larger microscope, the stage and objective rotate 

 together on a separate support, with focussing screw. The 

 ocular portion slides in its tube. The stage is graduated and 

 rotates, but there is no superstage. It is fixed in a vertical 

 position, and is not capable of inclination. The polariser is on a 

 crank arm, and has a weak converging lens over it. The analyser 

 fits over the eyepiece, which has cross wires and is not graduated. 

 A second eyepiece is provided, and three objectives, Nos. 3, 5, 

 and 7. 



63. Petrographic microscope. 



E. 69. 1888. Made by Seibert, Wetzlar. 



The polariser is on a sliding plate. Has a graduated rotating 

 stage, diaphragms, and converging lens. The optical tube 

 slides in a sheath, and the graduated fine adjustment is at 

 the top. The analyser fits over the eyepiece, and is coarsely 

 graduated. There are no slots in the side of the tube. The 

 instrument is fixed in a vertical position, and is not capable of 

 inclination. Two objectives and three eyepieces are provided, 

 with a second condensing lens. Selenite and quartz plate, and 

 three test objects. 



64. Petrograpliic microscope. 



1888. Exhibited by Watson Bros. 



English tripod stand ; body movable about a horizontal axis, 

 and with a stop to indicate when it is in a horizontal position. Has 

 a large polariser on a crank arm, graduated to 45 ; a circular 

 graduated glass-topped stage ; coarse and fine adjustment not 

 graduated. Analyser to fit over eyepiece, not graduated ; cross 

 lines to eyepiece ; has 1 in. and J-in. objectives. Condenser on 

 stand. Live box and forceps. 



65. Star microscope. Price, 61. 15s. 



1891. Exhibited by R. and J. Seek. 



Has a triangular base, with oblique support, with a horizontal 

 axis, about which the microscope rotates. Has a circular 



