Optical projection: a treatise on the use of the lantern in exhibition and scientific demonstration (1906)

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16 OPTICAL PROJECTION of the small blue sight-hole. For the lime-light, there ig usually a perpendicular slot sufficient for the passage back and forward of the tray-pin which carries the jets (figured in Chapter IV.) and enlarged at the bottom for the jet itself, and for the tray. Usually little light comes through these open- ings ; but with a powerful light it is unpleasant, and mars the effect. When this is found to be the case, two small brass eyes should be screwed into the back, near the top corners, into which drop the two ends of a wire bent as in fig. 9. On this semicircular wire is hung, by small rings or a broad hem, a curtain of black cloth, which quite stops the stray light, while allowing the jets to be got at readily. The top and chimney also claim a word. Paraffin lanterns have open tops, but, if also used for lime-light, a cover mus ^ be provided. Argands need a tall chimney ; but for lime-light lanterns such have quite gone out, and a short cowl on a bulged top is generally used, as shown in the tri-unial figured on page 119. If this has to be packed inside the lantern, it is very dan- gerous to the condensers, unless carefully wrapped in cloth ; therefore, if the lantern-box will not contain it in situ, the cowl should be so tapered that it will drop upside down into its flange or socket, which can also be reversed on the top of the lantern. It is very much better to make the top (always of sheet iron) flat, and the top of its quite shallow cowl also flat. This saves space and manipulation to begin with, but has a more important advantage. In experimental work it is often necessary to warm fluids and objects, and the flat heated top offers a convenient means for doing this. Even in slide exhibition, every exhibitor knows the difficulty en- countered from ' dew' upon his slides on a cold and foggy night. Such a flat top offers to him, also, a handy means of