618 THE POPULAR SCIENCE MONTHLY. 



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 pre- 

 viously 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 dis- 

 closes 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 car- 

 ried on in many establishments throughout the country and has 

 been reduced to very accurate practice. Three surfaces are util- 

 ized 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 tri- 

 angular prism. As the light is bent toward the common per- 

 pendicular 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 con- 

 sidered 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 con- 

 verge the rays while the concave ones disperse them. In design- 

 ing lenses for spectacles, these principles find application. If the 

 eye be perfectly formed, but have too little or too great convex- 

 ity, 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 grind- 

 ing 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 



