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 



