FIELD-GLASS

a short double telescope of which the two parts are the same in construction, and are placed parallel to each other so that the axes of the eye-lenses are at the same distance apart as the pupils of the observer’s eyes. In the Galilean tyre of field-gla , the objectives are convex and the eyepieces concave lenses, and the distance between these in either tube is, when least, equal to the difference of their focal lengths; this accounts for the shortne of the instrument. In a telescope with a convex eyepiece, the minimum lens distance is the sum of the focal lengths. By the use of 90° prisms, however, employing double-internal reflection, the rays which enter the objective traverse the tube three times before i uing through the eyelens. This hag the effect of shortening the instrument. Also the prisms cause & 430 FIELDING double-lateral inversion, which produces an erect image. The prismatic field-gla does not give such a bright image as the Galilean, partly on account of the Ic cf light by refiection at the gla surfaces, and partly because of the greater magnification employed.