. The book of photography; practical, theoretical and applied. instances the fraction ofan inch difference in the separ-ation of the component partsof a lens may make a strikingdifference in its optical quali-ties. It is therefore desirablethat the following details ofthe various stages throughwhich the lens has to pass,from the rough glass to thefinished objective, should becarefully studied. A gooddeal of useful information,likely to- be of value in ordin-ary photographic practice, willthus be gained. graphs have been obtained ofscenes from a play at nightby means solely of the ordinarytheat
. The book of photography; practical, theoretical and applied. instances the fraction ofan inch difference in the separ-ation of the component partsof a lens may make a strikingdifference in its optical quali-ties. It is therefore desirablethat the following details ofthe various stages throughwhich the lens has to pass,from the rough glass to thefinished objective, should becarefully studied. A gooddeal of useful information,likely to- be of value in ordin-ary photographic practice, willthus be gained. graphs have been obtained ofscenes from a play at nightby means solely of the ordinarytheatrical lighting. This lenshas hardly yet been broughtto perfection as regards criti-cal sharpness of definition anda few other factors, but it isby no means unlikely that itmay ultimately prove the start-ing point of a valuable ad-vance in the construction ofphotographic objectives. Theidea is, however, not entirelynew, for several early investi-gators have suggested or madeuse of a hollow spherical lensfilled with water for variousoptical or other Fi ?. 527.—3I.\CHiNE Shot (if Lkns Factouv. Lens Manufacture. Enough has probably been said of thevarious classes of lenses and the mainoptical principles regulating their con-struction and use. It will now be bothinteresting and instructive to consider the Exactness Required. There is piobably no other branch ofmanufacture which demands such pre-cision of workmanship as is necessary forthe production of photographic bricklayer and the tailor are proud 376 THE BOOK OF PHOTOGKAPHY. of working within an eighth of an inch,the cabinet-maker of working to ahundredth, the machinist deals in thou-sandths, the watch-maker in ten-thou-sandths, but the maker of photographiclenses works in hundred-thousandths ofan inch. The slightest failure in workingto this scale inevitably results in a de-fective instrument. Not only in the actualmanufacture, but in the designing and thecareful preparation of materials as well,experime
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