ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



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be desired, Although it is possible to form the blank by rolling or 

 pressing, the inventor of the process, W. J. Seymour, of the Merry 

 Optical Co., Kansas City, prefers to form the blank by blowing bbe 

 united bodies. He accomplishes this by gathering on the end' of a 

 blowpipe a body of glass of a certain kind, or index, oi colour, and by 

 then dipping it into a pot of molten glass of a different kind, index, or 

 colour. In the action of dipping care must be taken that the molten 

 glass moves progressively along the body, thereby washing away any 

 air-bubbles, foreign matter, or imperfections on the surface of such 

 body ; this, moreover, insures a perfect union between the two layers. 

 The body of glass, consisting of the two uniting layers, may be shaped 

 by blowing alone, but preferably by blowing in a suitable mould in the 

 manner well known in the art of blowing, so as to insuri regularity and 



12. 



Fig. 11. 



Fig. 13. 



. 14. 



uniformity of contour (fig. 11). After the shell, consisting of two or 

 more layers, has been produced a section or blank (shown by dotted 

 lines in figs. 12, 13, 14) is cut from the body of this shell. This section 

 is then ground so as to form a lens (shown in full lines in figs. 12, 

 13, 14) which is made up of a plurality of kinds or colours of glass. In 

 making bifocal lenses the blank is ground to the shape shown in full 

 lines, so that the different focal centres are placed in the desired parts 

 of the finished lens. Thus a lens is secured of as many focal powers as 

 desired, the number being dependent upon the number of glass layers 

 of different kinds, index, or colour, which have been formed into one 

 integral body, the arrangement of the focal centres being adjusted by 

 the grinding process. It will be understood that in grinding a bifocal 

 lens from a blank which has been formed by this process one or more 

 of the layers of which the blank is composed may be cut entirely away, 



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