. The book of photography; practical, theoretical and applied. camera, lighting ar-rangement, etc., being fitted up in theirproper positions for taking a photograph,a metal mask is placed at the back of thecamera, the black velvet hood is drawndown over the head, and one of the screensis fixed by springs against the plain glassscreen. If a disc of light is seen shiningthrough the coloured screen, the next higher one is put in place of it until finallyonly the very dimmest light is seen topass, while the next higher screen sub-merges it altogether. The number ofthicknesses of paper in the scree
. The book of photography; practical, theoretical and applied. camera, lighting ar-rangement, etc., being fitted up in theirproper positions for taking a photograph,a metal mask is placed at the back of thecamera, the black velvet hood is drawndown over the head, and one of the screensis fixed by springs against the plain glassscreen. If a disc of light is seen shiningthrough the coloured screen, the next higher one is put in place of it until finallyonly the very dimmest light is seen topass, while the next higher screen sub-merges it altogether. The number ofthicknesses of paper in the screen whichallows only the very faintest light to passis a measure of the intensity of the light. Mathematical Factors Involved. If the intensity of the light be repre-sented by In and the fraction which pene-trates each film by f, the intensity afterpassing one film is I x f ; after 2 films,In X f X f, or In f- ; after 3 films, I„ f^ ;and after n films, In x f. If this is thefinal point, then I„ x f°= a very faintlight = In, say. Similarly with light of a. Fig. 682.—Warnerkes Sexsitometer intensity Im requiring m layers, In X fn = Iiu X fm whence (I) InIm L- — I 111 — nfn ~ and since the exposure necessary to pro-duce equal density is inversely as theintensity of the light Em _ £ > - nEn ~ f = m
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectphotography, bookyear
