1895. ] NEW YORK ACADEMY OF SCIENCES. 165 
the screen from the prism was then adjusted until the opposite 
edges of the two images exactly coincided. A Nicol prism B, 
capable of rotating about its axis and furnished with a graduated 
circle and vernier for reading azimuth to a tenth of a degree, 
constituted the only other essential part of the instrument. A 
small telescope C was used for viewing the images of the rec- 
tangular opening O. The instrument was mounted upon a sup- 
port G, furnished with a horizontal axis at F, about which the 
upper portion of the apparatus could be revolved. The axis of 
the tube bearing the double prism and Nicol could thus be in- 
clined so as to make any desired angle with the vertical. Since 
the two images furnished by the double prism consist of light 
polarized in planes at right angles to each other, the rotation of 
the Nicol willevidently extinguish each of them in turn. There 
will be four extinctions in the course of a complete revolution of 
the Nicol, and between any two extinctions there isa point for 
which the images, as seen through the Nicol, have exactly equal 
intensities. If now a partially polarized beam is under exami- 
nation, and if, the plane of polarization of this beam being 
known, the principal sections of the prism are set parallel and 
perpendicular to this plane, that position of the Nicol which 
equalizes the two images, evidently defines the relation between 
