The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Specific Gravity                                     Spherical Aberration
For Making any Quantity of Solution when stated in Parts per iooo, loo, etc.
We may say that, in giving the above figures, the resulting solution is sufficiently correct as regards percentage composition, though it may measure slightly more than the water taken, owing to the increase in volume which always takes place in some degree when a solid passes into a solution in a given amount of liquid. This expansion is not appreciable for small amounts of the solid, say up to 5 per cent., but at 25 per cent., or more, it may be very noticeable.
Specific Gravity. See Hydrometers and Hydrometry.
Spectacle Lens. See Monocle and Lens.
Speed of Plates. See Exposure.
Spherical Aberration. When a simple convex lens and short focus is used to project an image on to a focussing screen it will be found that the image is nowhere absolutely sharp ; and the reason of this indistinctness is spherical aberration. Let L, fig. 121, represent a short-focus convex lens, and r'r'r r repre-sent rays of light striking the lens ; the marginal rays R r cross the axis of the lens or come to a focus ata point nearer the lens than the central rays, r' r' ; so that if an image atbe examined it will be seen to be surrounded by an aureola or ring of light, ad, which is formed by the spreading of the marginal rays after
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