The Philosophical magazine; a journal of theoretical, experimental and applied physics . uard against a false ratioof intensity of light due to the lens, negatives of the star weretaken at different parts of the plate, and a mean taken. As thelens used was non-distorting and of long focus, the edge andcentre of the plate, when directed towards the sky or on a uni-formly white surface, had sensibly the same illumination. Eachportion of the strip cut from the negative whose opacity wasto be compared was placed above the wedge, at B, and oppositethe slit C. These were clamped together and moved t
The Philosophical magazine; a journal of theoretical, experimental and applied physics . uard against a false ratioof intensity of light due to the lens, negatives of the star weretaken at different parts of the plate, and a mean taken. As thelens used was non-distorting and of long focus, the edge andcentre of the plate, when directed towards the sky or on a uni-formly white surface, had sensibly the same illumination. Eachportion of the strip cut from the negative whose opacity wasto be compared was placed above the wedge, at B, and oppositethe slit C. These were clamped together and moved till lightfrom behind, shining through the slit and through the imageand the wedge respectively, appeared of the same brightness on M2 164. On the Opacity of the Developed Photographic Image. the ground glass. The position of the slit in regard to the scalewas noted, and the intensity of light transmitted calculated bythe ordinary formula. Each strip was compared six times—three times by myself, and three times by an assistant. A meanof the six readings was taken as correct. Fi-. F — 1; A and B are the curves given by the images on the dry plates. C and D are the curves given by the image on wet plates. The dotted lines indicate the line whose ordinates give an arithmeticprogression of transparency, E F being unity or transparency. F G represents the length of the strips examined, and therefore thevarying intensity of light, F being zero and G the maximum. Regarding the curves given by the dry plates^ if we sup-pose that varying intensities of light cause a corresponding re-duction of the bromide of silver after development, it can beeasily demonstrated that the intensity of light passing throughthe image after clearing away the unaltered bromide would be l = ^^ («) where n and k are constants depending on the thickness andopacity of the bromide film, and I the intensity of the light ^?-«-duci/ig any one part of the image—that is, on the suppositionthat the image
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