436 Johannsen — Some Simple Improvements for a 



entirely closes the upper part of the opening in the tube and 

 prevents all reflection from the surface of the nicol. The 

 lever c is shown at c\ rotated through 90°. 



#. A permanently attached combination ivedge. — A great 

 deal of time is ordinarily lost in picking up the accessories 

 to the microscope and in hunting for the slot into which they 

 are to be inserted. The simple contrivance shown in figure 2 



Fig. 2. 



0{ 1 a J b ■ | c T^O 



has been found to overcome all this. A carriage, exactly fit- 

 ting the slot above the objective, is inserted in the tube of the 

 microscope and is kept in place by two end screws like those 

 holding the Bertrand lens bar. At one end is a square of 

 gypsum (a) giving red of the first order ; b is an opening ; and 

 c is a quartz wedge underlain by a mica plate, the two minerals 

 having their c directions at right angles to each other, and sim- 

 ilar in construction to a Wright quartz gypsum-wedge. The 

 thickness of the underlying mica plate is so chosen that it 

 exactly compensates the front end of the quartz wedge, con- 

 sequently the colors of the combination wedge begin at dark- 

 ness and gradually increase to the fourth order as the wedge is 

 shoved forward. A spring (S) attached to the side of the 

 microscope tube presses against the carriage and produces 

 enough friction to hold it wherever it is placed. When 

 the opening b is centered, the spring drops into a rounded 

 notch as shown. Upon the upper side of the carriage a 

 scale is engraved and the end of the spring shows the order 

 of the color at that time beneath the cross hairs of the 

 microscope. 



3. A rotating lower nicol for observing very slight pleo- 

 chroism. — In the ordinary petrographical microscope, such as 

 the Fuess III-a, which is the one generally used, the arrange- 

 ment for rotating the lower nicol is extremely crude, and the 

 force necessary to overcome the friction by which it is retained 

 in place makes smooth rotation impossible. In order to 

 observe very slight pleochroism, it is often desirable to turn the 

 nicol instead of the stage ; the movement of the mineral in the 

 latter case causes confusion and makes it impossible to observe 

 very slight changes of color. Nor is it possible to obtain the 

 same results by removing the lower nicol and rotating the 

 upper nicol in its place, for a distortion of the image results 

 and consequently, also, a movement of the lines. For this 



