The luminous rays coming from a distant object are received upon an inclined mirror, which is elevated above the parapet of a fortification, and are reflected downwards to a second, which is placed at a corresponding angle. If necessary, lenses can be interposed, so as to give a magnified view of the distant object that is being examined. By means of such an instrument, the movements of the enemy can be followed without danger, the apparatus being generally of small size, and not attracting notice. Amongst the varieties of this instrument, is one whose use is readily seen by inspecting fig. 61, by which it seems to be perfectly possible to see with safety all that is going on outside the door of the house without being perceived. The line of the mirrors in this case is at right angles to that of the polemoscope in fig. 60. Amongst the different varieties of polemoscope which have been invented, may be mentioned a reflecting opera-glass, which was greatly used by the beaux and dandies of the last century. In the tube of this instrument was inserted an inclined mirror, which allowed the spectator to point his glass in quite a different direction to that of the object he was really looking at. In fact, it was constructed somewhat on the same principle as the Herschellian or Newtonian telescope, and enabled the possessor, while apparently enjoying the play, to observe all that was going on in the boxes or pit of the theatre. Years ago, there was a little instrument of a similar kind, sold for a penny in the streets of London, which consisted of a morsel of looking-glass set at an angle, in a pill-box, and which gave the possessor the power of seeing all that was going on behind him. Persons who wear dark preservers are often in the habit of observing all that is going on behind their backs by the reflection seen in the corner of their glasses. Such are the principal optical recreations founded on the reflecting and refracting properties of mirrors and lenses. We shall end this chapter by appending to it the description of a few additional optical amusements that are quite within the reach of the amateur.