G. H. Williams — Petrographical Microscope. 117 



with a graduated silvered circle and index. It is held by a 

 cylindrical frame in which it may be raised or depressed at will 

 by a rack and pinion movement. This frame is attached to 

 the under side of the stage by a swinging arm, so that the 

 whole |3olarizing apparatus may be thrown to one side if de- 

 sired. A strong compound lens may be screwed upon the 

 upper end of the polarizer whenever strong illumination or 

 converged polarized light are needed. 



The circular stage (9*5 cm. in diameter) is provided with a 

 beveled silvered edge, graduated to degrees. Upon this is 

 mounted for smooth and concentric revolution the admirable 

 mechanical stage, known in the manufacturer's catalogue as 

 No. 1052. This carries an index for reading the graduated 

 circle, and is also provided with silvered graduations for its two 

 rectangular movements, whereby any point in a section can be 

 readily located. The upper sliding bar which carries the 

 object has been shortened so as to be only flush with the re- 

 volving stage when pushed to its extreme limit on either side. 

 With this, square or short rectangular glasses must be used for 

 mounting which will avoid any interference with the revolu- 

 tion of the stage. 



Into the nose-piece, just above the objective, is an opening 

 intended to receive the four following accessories, each mounted 

 in a separate brass frame : (1) a Bertrand lens for magnifying 

 the interference figures ; (2) a quarter-undulation mica-plate ; 

 (3) a quartz wedge ; (4) a Klein quartz-plate or a gypsum 

 plate with red of the first order. 



The centering of the various objectives is secured by two 

 screws having motions at right angles to each other. 



The upper Nicol-prism or analyzer is inserted in the tube in 

 order to avoid the diminishing of the size of the field which is 

 unavoidable when the prism is placed over the ocular as a cap. 

 To accomplish this, and at the same time to keep the tube 

 dust-tight, the nicol is enclosed in one side of a double cham- 

 bered box. The other side is left vacant and the box may be 

 slid to and fro according as ordinary or polarized light is de- 

 sired. A metal sheath protects this box from above. 



The microscope as here described in a case with a single eye- 

 piece, but without objectives, may be obtained for $108.00. 

 With two eye-pieces (one with cross-hairs and the other with 

 micrometer) and two objectives (f and \ inch) its cost is $135.00. 

 The cost of a solid brass stand is about $25.00 more. 



The instrument as here figured is less expensive than the 

 importation of the lower grades of European petrographical 

 stands ; and considerable practical experience with it has 

 shown that it renders decidedly better service. 



Petrographical Laboratory, Johns Hopkins University, November, 1887. 



