. The half-tone process. A practical manual of photo-engraving in half-tone on zinc, copper, and brass. o £16, according to the diameter of the lens andthe reputation of the firm it is bought from, this priceincluding the fitting in a brass or aluminium box, whichis usually screwed on to the front of the lens, as shownin fig. 25. Scientifically speaking the hypotenuse of a rightangle prism should be a perfect reflecting surface, andtheoretically it is held that it requires no , however, does not work out in practice, as an un-silvered prism gives a prismatic band down one sideof
. The half-tone process. A practical manual of photo-engraving in half-tone on zinc, copper, and brass. o £16, according to the diameter of the lens andthe reputation of the firm it is bought from, this priceincluding the fitting in a brass or aluminium box, whichis usually screwed on to the front of the lens, as shownin fig. 25. Scientifically speaking the hypotenuse of a rightangle prism should be a perfect reflecting surface, andtheoretically it is held that it requires no , however, does not work out in practice, as an un-silvered prism gives a prismatic band down one sideof the sensitive plate, and this comes out a dark streakon the negative. When the prism is silvered thistrouble entirely disappears. The reason of this neednot be entered into here. Suffice it to say that allprisms in practical use are silvered, and once silveredthey rarely need doing again. Thus, the great advan-tage that can be claimed for the prism is that it is apermanent reflector, whilst the mirror is very suscep-tible to tarnish, and constantly needs silvering, which 68 THE HALF-TON E is too delicate an operation to be done at home. Itis contended that a prism is slower than a mirrorbecause there is a loss of light by absorption in thethickness of the glass and by reflectionat its surfaces, but when it is remem-bered that a mirror can never bemaintained in sufficiently goodcondition to make this com-parison possible such acontention can have littleweight. Certainly theprism is, to my mind,\ the best possible thingd ito use if the worker canafford it, and it isespecially convenientin the colonies, or elsewhere abroad where the meansof getting mirrors resilvered are not ready to hand. An optical plane mirror costs about one shilling persquare inch. This article is a piece of the finest plateglass, about j^ths of an inch thick, ground down onone side until it is optically fiat. It is then silveredon the surface—, the side which has been opticallyworked—not o
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Keywords: ., bookcentury1900, bookdecade1900, bookidhalftoneproc, bookyear1904
