G2 TRANSACTIONS OF THE [pxEC. 12, 
from near Clear Creek, Wyoming Territory, weighing twenty- 
five and three-fourths pounds. 
December 12, 1887. 
STATED MEETING. 
The President, Pror. J. S. NEWBERRY, in the chair. 
Fifty-four persons present. 
An invitation to the decennial anniversary of the Engineers’ 
Club of Philadelphia, on December 17, 1887, was read; also 
an announcement from the Smithsonian Institution of the elec- 
tion as Secretary of Samuel P. Langley. 
Pror. W. LECONTE STEVENS exhibited and described 
NEW APPARATUS FOR DEMONSTRATING THE LAWS OF REFLECTION 
AND REFRACTION. 
Various devices are in use for this purpose, the best known of 
which is the circular tank introduced by Tyndall, the lower half 
of which is filled with water rendered slightly cloudy in order 
to trace the path of the refracted ray in it, while in the upper 
half is smoky air through which the path of the incident ray is 
to be traced. The lantern beam is deflected obliquely downward 
by means of a mirror so as to enter the water at the centre of 
the circle. Because the surface of the refracting medium must 
remain level, a number of inconvenient adjustments are required 
whenever the angle of incidence is varied. The index of refrac- 
tion of water being only 1.33, the deviation of the ray on enter- 
ing it is not very striking, and the cloudiness of the water often 
constitutes an obstacle rather than an aid in demonstration, be- 
cause the deflected beam becomes diffused at a very short dis- 
tance below the surface. 
In the present apparatus these difficulties are allavoided. The 
refracting medium is a hemicylinder of polished glass, mounted 
so asto turnon a horizontal axis. The radiusis 2.5 cm., the 
length 5cm. The axis passes through the centre of a circle of 
white cardboard whose radius may be 20cm. or 30cm. Each 
of its quadrants is graduated from 0° to 90°, as shown in the 
accompanying cuts, the diameter connecting the two zero points 
being perpendicular to the diameter of the hemicylinder of 
glass. A beam of light is sent through a horizontal slit in front 
