called a "range-finder" which soldiers and sailors use to find the distance of the enemy, they manage to use their eyes just as though they were on long horns.

In Fig. 12 A and B represent the real eyes of

the observer, close together; C and D his artificial eyes wide apart. They are two lenses C and D, with mirrors behind them G and H: and these artificial eyes can be turned on the object by screws, just as our eyes can be turned by our muscles. Rays of light EC from the distant object falling on C are reflected by the little mirror behind it so as to travel