Page:Popular Science Monthly Volume 39.djvu/636

618 A noted French critic has left on record a touching account of the first time he ever looked through a pair of spectacles. He was terribly near-sighted, but no one had ever given sufficient attention to the defect to make any attempt to remedy it. One day, while still a boy, he got hold of his grandfather's spectacles, and put them on. Great was his surprise to find that the giant tree which shaded his play-ground was made up of individual and beautifully formed leaves instead of being, as he had previously supposed, one almost solid mass of green foliage. The boy fairly danced with delight, for a new world was suddenly opened to him.

A little fragment of glass, which thus gives sight to the almost blind, must claim attention even before the instrument which discloses either microcosm or macrocosm, for it has to do with the most important of all sciences, the science of daily living. The grinding of the small lenses for spectacles and eye-glasses is carried on in many establishments throughout the country and has been reduced to very accurate practice. Three surfaces are utilized—spherical curves, cylindrical curves, and prismatic faces. Their effect can readily be understood if one will consider for a moment the passage of a beam of light through an ordinary triangular prism. As the light is bent toward the common perpendicular on entering the glass, and away from the common perpendicular on leaving the glass, the total bending will be toward the base, or thicker portion. Now a lens may be considered a double prism; convex, if the two prisms be placed base to base, and concave if they be apex to apex. Since the light is always bent toward the thicker portion, the convex lenses converge the rays while the concave ones disperse them. In designing lenses for spectacles, these principles find application. If the eye be perfectly formed, but have too little or too great convexity, the remedy is found in glasses with simple spherical faces; but if the structure of the eye be faulty and non-symmetrical, as in the astigmatic, the glasses must have cylindrical or prismatic surfaces.

The bit of glass to be formed into a lens is fastened by means of pitch to a small block of hard rubber so that it may be more readily handled. It is ground by being pressed against a rapidly revolving metal tool, whose curvature is equal and opposite to that desired in the lens. This is known as the "rough tool" and is made of cast iron. It is mounted on a vertical spindle, and is kept moistened with emery and water. Several grades of emery are used in succession, changing from coarse to fine as the grinding proceeds. As a result of this process the glass has a rough surface and is no longer transparent. It is now transferred to the "fine tool." This is made of brass and has its surface as true