. The book of photography; practical, theoretical and applied. , and the sensitive plate used forthe photograph would have to be exposedfor a great length of time. If the hole isenlarged, more light is obtained, and if asuitable lens is placed in it, all the rays oflight meet at a point, as shown in Fig. 39,which illustrates the simplest form of lens. Different Forms of Simple Lenses. Simple lenses from which photographicand other combinations are built up areshown in section by Fig. 40, in which A isa double - convex or bi - convex ; b is a plano-convex; c is a meniscus; D is adouble-concave
. The book of photography; practical, theoretical and applied. , and the sensitive plate used forthe photograph would have to be exposedfor a great length of time. If the hole isenlarged, more light is obtained, and if asuitable lens is placed in it, all the rays oflight meet at a point, as shown in Fig. 39,which illustrates the simplest form of lens. Different Forms of Simple Lenses. Simple lenses from which photographicand other combinations are built up areshown in section by Fig. 40, in which A isa double - convex or bi - convex ; b is a plano-convex; c is a meniscus; D is adouble-concave or bi-concave ; E is a plano-concave ; and F is a concavo-couvex, some-times incorrectly termed a concave-meniscus. TSe Single Lens. The photographers single lens (Fig. 41)consists of two or more glasses cementedtogether and looking like one, fixed in abrass tube or mount ; the negative ele-ment is of crown glass and the positive offlint glass cemented together with Canadabalsam. The lens must be fitted with stopsof some kind ; the simplest form of stop is a. Fig. 41.—Single Lens with Iris Diaphragm. circular plate containing variously sizedholes, which may in turn be brought beforethe centre of the lens. The stop may be aniris diaphragm, consisting of a seriesof overlapping plates, which, by turning aling surrounding the lens mount, causesthe hole to get gradually smaller ; or it maybe a number of little plates with differentsized holes which drop in through a slot,these being called Waterhouse dia-lahragms. They all give equally good re-sults, but the iris is the more con-venient. In front of this comes a project-ing ring called the hood, and at theback is sciewed a metal flange, withthree holes for attaching to the camerafront by small screws. These lenses aresuitable for landscape work or an occa-sional portrait in bright light, when theyshould be used with full aperture orthe permanent stop. Single lenses givedistortion, causing, for instance, square 32 THE BOOK OF PHOTO
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