377 
periscopic Camera Obscura and Microscope. 
tinctly without the use of a concave glass, it may be assisted 
in that respect by a due degree of concavity given to either, 
or to both the transmitting surfaces of the prism. It is, how- 
ever, to the upper surface alone that this concavity is given ; 
for since the eye is then situated on the side toward the centre 
of curvature, it receives all the benefit that is proposed from 
the periscopic principles. 
