1887. ] NEW YORK ACADEMY OF SCIENCES. 73 
of the lantern, transmitted through the glass at the middle, and 
focussed upon the poinf A (Fig. 1), the plane surface of the 
hemicylinder being perpendicular to the beam. ‘The incident, 
reflected, and transmitted beams are all in the same horizontal 
line. The cardboard being slightly turned toward the lantern, 
the path of the beam on it is easily seen with good definition. 
The room should be but half dark, so that the graduation can be 
read on the circle. Turning the hemicylinder now through any 
desired angle, such as 50°, the cardboard moving with it, part 
of the beam is reflected and part refracted ; both beams are 
