Early Work in Photography (Paperback)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1896 edition. Excerpt: ...character, and its violet rays, which are more refrangible than the yellow, will be focussed, and will give a sharp image at a point nearer the lens. As the violet light rays which are dark to the eye are those which are most active photographically, while the yellow light rays (brightest to the eye) are least active we find that we have here two foci (explained later) one called the visual, and the other the chemical or actinic focus. If we place our ground glass screen (or later our sensitive plate) at the point where the visual image is strongest and sharpest, we shall find, on developing the photographic image, that it is feeble..and blurred. If we move the screen (and plate) forward until the visual image is somewhat blurred, we shall find that the photographic image, on development, is better defined. This class of truly single'lens is s)metimes used by photographers of the impressionist school, and they find it necessary, after focussing their image on the screen, to rack in the camera back to an extent ascertained by experiment. Spherical aberration is rather more difficult to explain. In the last diagram we shewed all the rays from (say) the tip of candle-frame, coming to one point on the ground-glass, whether they passed through the centre or the margin of the lens. But this diagram was falsely drawn, merely to illustrate a point in passing; and what actually happens would be better represented by the above diagram, which is, however, grossly exaggerated. It will be seen that the rays passing through the centre of the lens pass through or focus at the point F, while those passing through the edges of the lens focus at F', with t e result that the images formed by the two sets of rays do not coincide; and, as every point in..