WORKING BY ARTIFICIAL LIGHT.
The best form of apparatus for this work differs only qlightly front that described, for in it the construction usually tends to hardness of outline, if is to be avoided rather than otherwise. The two favourite illuminants are incandescent gas, with which tl,c exposure is short and the definition crisp with a minimum of hardness, and the paraffin lamp, with softer outlines are produced, but which has the great disadvantage of ex cessive heat and a disagreeable odour. The dangers of injury are, of course, greater in the ease of the incandescent gas flame owing to the fragile nature of the mantle. The apparatus shown in Fig. 432 is one of the most popular on the market. It is designed on the cantilever principle, and consists of an oil or gas illuminant adjustable to the condenser, in larging with the lantern. The phenome non of producing an image greater in ' dimension than the object is nut necessarily due to any magnifying power possessed by the lens, hut merely to the fact that rays of light travel in straight lines, and that those travelling in con verging directions will cross and spread out again and therefore the longer the distance over which they have to travel the more tbey will have spread out, and consequently the larger the image pro duced. These remarks apply equally
whether the rays pass through a small hole, or whether they pass through the lens. Therefore the size of the image is dependent upon the proportionate distance between the receiving surface and the hole, and the object and the hole, front of which is a framework carrying the negative, which slides in on a earl.' Y. The bellows is racked in and out by inea,. of brass rods passing into tightly fittii g tubes. A swing front is provided, which is useful for correcting distortion in th negative. Another excellent lantern, ob tainable for use with electricity, limelight, incandescent gas, acetylene, or oil, is shown by Fig. 433. Fig. 434 illustrates a convenient and portable form for use witl oil ; the lamp, as will be seen, packs away inside.