This is an instrument 2 in. diameter by 3/4 in. thick, to be held in the hand and arranged as shown in the accompanying figure, in which A is the sight hole where the eye is placed, B and c are openings in the rim through which rays of light can enter from poles at D and E, only farther off; F is a glass half silvered and half plainthe junction line being in the plane of the instrument: G is a whole mirror. In using the instrument for sighting poles as shown, it would be held in the left hand; with the eye at A, the pole D would be seen through the opening B and the plain part of the glass F; the observer being at the point where a right angle would be measured between D H, E H. Rays of light from pole E will reach mirror G and be reflected from there to the silvered part of glass F and thence to the eye at A, so that the glass F will appear like Fig. 2, the piece of pole J seen by direct vision being exactly over the piece of pole K seen by reflection. If in using the instrument the poles do not coincide, the station of the observer must be shifted until they do, or as an alternative one of the poles must be shifted. If the poles appear to coincide at the junction of the glasses but not to be in a straight line, it will he due to one of the poles being at a higher level than the other.

Fig. l.

Fig. 2. An Optical Square.