Johannsen — A Petrographical Microscope. 435 



Art. XXXIX. — Some Simple Improvements for a Petro- 

 graphical Microscope / by Albert Johannsen. 



1. A rotating upper nicol in which the annoying reflection 

 of light fr vm the surface is overcome. — In examining, between 

 crossed nicols, minerals which are rather dark, a small amount 

 of light falling upon the upper surface of the nicol produces 

 a hazy appearance of the image. Figure 1 represents a light- 



Fig. 1. 



^TTO 



tight modification of the upper nicol of the Fuess III-$ micro- 

 scope. A vertical section through the carriage is shown at A. 

 A rotating collar {a a') is moved by the lever c and is supported 

 by the flanges b V of the outer tube in which it rotates. A 

 part of the scale (d) is attached to the box and is divided into 

 degrees, although only the 10° divisions are shown at B. 



B and C are respectively the horizontal and the vertical 

 projection. The separation of the scale into two parts, which 

 was made necessary by its lowered position, is shown at 

 e and /. The slot </, which is also shown in figure 1-C, is for 

 the easy removal of the nicol from the carriage. The prism is 

 rotated 90° and is lifted out, after the entire carriage is taken 

 from the tube of the microscope, by the removal of the 

 screw i. Figure 1-D is an end view showing how the plate m 



