2nd.—Inside " f 3 convex,
crown.
3rd.—Outside " f 3 concave,
4th.—Inside " 4f convex,
flint.
76.—By different makers the surfaces are changed as far as reversing the curvature of the front glass, and indeed very good glasses are made with the 1st, 2nd and 3rd = (f/2·5). In all cases true convergence of the white ray is only obtained by correction of the outer and inner surfaces, or by figuring, as it is technically termed, in which the curvature is not only made greater or less, but its character is altered generally in the direction from circular to elliptical section. The qualities of the object-glass cannot be over-estimated by the practical surveyor. A heavy instrument with inferior object-glass may be carried about for years, whereas a lighter instrument with good object-glass would perform better work. Excellent information upon this subject was given in a lecture before the Royal Institution by the eminent optician, Sir Howard Grubb, of Dublin.
77.—Optical Arrangements of the Telescope.—The earliest form of telescope is that of Kepler, Fig. 13. In this the rays from the object-glass cross in front of the eyeglass; consequently, the image is inverted. This form is at present little used except in combination with a separate eye-piece.
Fig. 13.—Kepler's telescope.
Fig. 14.—Galileo's telescope.