American Petrographical Microscopes. — Winchell. 229 
structed under the direction of Prof. Geo. H. Williams, of 
Johns Hopkins University, and was described by him in the 
Am. Jour. Sci., February, 1888. It is illustrated by the ad- 
joining figure, one-third its actual size. It was first exhibited 
and described by him at the winter meeting (Dec. 1887) of the 
Society of Naturalists, at New Haven, Ct. Following is 
his description.^ 
Coming now to the peculiarly petrographical features, we have the 
lower Nicol-prism or polarizer enclosed in a cylindrical metal box, both 
ends of which are protected by glass. This box is capable of a complete 
revolution, and is provided 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 move- 
ment. This frame is 
attached to the under 
side of the stage by a 
swinging arm, so that 
the whole polarizing ap- 
paratus may be thrown 
to one side, if desired. 
A strong compound lens 
may be screwed upon 
the upper end of the 
polarizer whenever 
strong illumination or 
converged pol a r i z e d 
light is needed. 
The circular stage (9.5 
cm. in diameter) is pro- 
vided with a beveled 
silvered edge, graduated 
to degrees. Upon this 
is mounted for smooth 
and concentric revolu- 
tion the admirable 
mechanical stage known 
to the manufacturer's 
catalogue as No. 1052. 
This carries an index for 
reading the graduated 
circle, and is also provi- 
ded with silvered grad- 
uations for its two rec- 
tangular movements, 
whereby any point in a 
,„,. section can be readily 
^^^ located. The upper 
sliding bar which carries 
the object has been 
Fig. 6. Bausch-Lomb Lithological Stand.' shortened so as to be 
only flush with the revolving stage when pushed to its extreme limit 
on either side. With this, square or short rectangular glasses must be 
3 Am. Journal of Science. February, 1888. p. 116 
