666 THE AMERICAN NATURALIST. [Vor. XXXV. 
a glass plate, P, placed in front of the V, parallel to the board, 
and secured to it by means of a block, W, and the other is a 
ruled scale attached to the board. The lines on the second 
QD 
scale are directly behind those on the glass plate and, there- 
fore, are not seen in the figure. The two scales are exactly 
alike, except that one is etched on glass and the other ruled on 
