This chapter is necessarily sketchy and not in the least intended to give the reader a complete account of technical glass manufacture, far less of the intricate and almost incommunicable art of making objectives and mirrors. It may however lead to a better understanding of the difference between the optical glass industry and the fabrication of commercial glass, and lead the reader to a fuller realization of how fine a work of art is a finished objective or mirror as compared with the crude efforts of the early makers or the hasty bungling of too many of their successors. For further details on making, properties and working of optical glass see: HOVESTADT: “Jenaer Glas.” ROSENHAIN: “Glass Manufacture.” SIR HOWARD GRUBB: “Telescopic Objectives and Mirrors: Their Preparation and Testing.” Nature _34_, 85. DR. HENRY DRAPER: “On the Construction of a Silvered Glass Telescope.” (Smithsonian Contributions to Knowledge, Vol. 34.) G. W. RITCHEY: On the Modern Reflecting Telescope and the Making and Testing of Optical Mirrors. (Smithsonian Contributions to Knowledge, Vol. 34.) LORD RAYLEIGH: Polishing of Glass Surfaces. (Proc. Opt. Convention, 1905, p. 73.) BOTTONE: “Lens Making for Amateurs.” CHAPTER IV THE PROPERTIES OF OBJECTIVES AND MIRRORS The path of the rays through an ordinary telescope has been shown in Fig. 5. In principle all the rays from a point in the distant object should unite precisely in a corresponding point in the image which is viewed by the eyepiece. Practically it takes very careful design and construction of the objective to make them meet in such orderly fashion even over an angular space of a single degree, and the wider the view required the more difficult the construction. We have spoken in the account of the early workers of their struggles to avoid chromatic and spherical aberrations, and it is chiefly these that still, in less measure, worry their successors. [Illustration: FIG. 47.—Chromatic Aberration of Convex Lens.] The first named is due to the fact that a prism does not bend light of all colors equally, but spreads them out into a spectrum; red refracted the least, violet the most. Since a lens may be regarded as an assemblage of prisms, of small angle near the centre and greater near the edge, it must on the whole and all over bend the blue and violet rays to meet on the axis nearer the rear surface than the corresponding red rays, as shown in Fig. 47. Here the incident ray _a_ is split up by the prismatic effect of the lens, the red coming to a focus at _r_, the violet at _v_. One can readily see this chromatic aberration by covering up most of a common reading glass with his hand and looking through the edge portion at a bright light, which will be spread out into a colored band.