50 OPTICAL PROJECTION 



28. Adjusting Oxy-gas Jets, These forms of jet will 

 easily give 200 candles, and with care have reached 250 

 candles, and more. In many cases, however, they never give 

 what they might for want of a proper adjustment, which is a 

 matter of more nicety with them than with mixed-gas jets. 

 The same remark applies to oxy-spirit jets, and the adjustment 

 is practically the same in both cases. 



The most general error is to use too much weight on the 

 bag. It seems considered the correct tiling to put on a 56-lb. 

 weight in any case, but in many instances 28 Ibs., or only a 

 few Ibs. beyond, will give a better light, and the too fast rush 

 of the oxygen through the gently-emitted hydrogen actually 

 cools the lime. The distance of the lime from the jet is also 

 all-important. These points cannot be adjusted in the lantern, 

 as every change in position there affects the optical adjustments 

 also, and thus disguises the result as regards brilliance. The 

 jet should therefore be taken out of the lantern, and placed 

 so that its naked illuminating effect upon the screen or a wall 

 can be seen. Then experiments should be deliberately made : 

 first as to oxygen, and then as to the distance of the lime 

 from the orifices. This will probably not differ much either 

 way from half an inch ; but it will be found that an eighth of 

 an inch will make a great difference. At each change in 

 distance, however-, 3t may be found that a different flow of 

 the gases gives a better light at that distance ; and it has to 

 be ascertained what distance, properly adjusted by tap and 

 pressure for its proper flow of gas, gives the best light. That 

 is the adjustment for this particular jet, and, once made, is 

 made once for all. It is, therefore, worth while to take 

 trouble over it. 



What are sold as ' soft ' limes are best for oxy-gas as well 

 as spirit jets, but ' excelsiors ' do also very well. The lime is 

 little acted upon by the comparatively moderate heat, and a 

 lime-spindle which can be turned by a milled-head underneath, 

 as in fig. 81, will answer if economy is desired. But any of 



