LEFT  SEARING A

rubber guard is fitted round the eye-pieces . Its functions are to guide the eyes of the observer into the correct position, and to protect them from side light and the distressing effect of wind . It also guards the forehead against the jar occasioned by firing heavy guns . The upper portion of the field presented to the left eye is used as a finder, the lower portion is occupied by the scale upon which the ranges are engraved . The finder is a low-power telescope of large field, to the centre of which the objective is brought . When the telescope is thus correctly aligned, the objective will be seen with the right eye largely magnified, but as two partial images separated by a thin black horizontal line . When coincidence of the images is effected by means of the working head, the range can be 2 The length of tube varies from 3 ft. in the smaller to 9 ft. in the larger instruments . i read off against a pointer from the scale seen with the left eye . For 1 or on different parts of the same objective, and thus inaccuracy in the night use, means are provided for illuminating the scale . The range recorded-range must result . The instruments are expected to give an to lights may be ascertained by the use of the astigmatizer, an optical accuracy of less than device by which a point of light is drawn out into a vertical streak . 2 % at 2000 yds .

For A*,ogjectioe A

beam of light from the objective falls on each reflector (fig . 2), ranges over that distance, i.e. for usual DOW uFPORT /IMDER OBJECT GLASS SUIMORT WINDOW RE n artillery ranges, it is IRATE WIN ) FRAME EYE PIECE PRIS5S-W SCALE ass desirable to use a ccro OBJECT GLASS CASE >rZn 'redouble base (1oo yds .