42 OPTICAL PROJECTION 



CHAPTER IV 



THE LIME-LIGHT 



IN one or other of its various forms, the lime-light is far more 

 largely employed than any other for serious lantern-work of 

 all kinds, and is only likely to be superseded by such advances 

 in electric lighting as may make an electric current avail- 

 able in public buildings generally, without burdening the 

 lecturer with the provision of generating apparatus. It is 

 cheap, easily managed, and perfectly free from danger when 

 its principles and methods are understood. 



Till very recently, it was the almost universal practice to 

 use the oxygen from a gas-bag under pressure, and this 

 method is and will be still so general, that it is convenient to 

 begin with it. 



24. Gas-Bags. These are of three kinds. A thin kind, 

 made of jeanet treated with india-rubber solution, has scarcely 

 any wear, and even for low pressure such are bad economy. 

 Much better are those usually known as ' best black twill,' and 

 generally used. It pays best to purchase these for all forms 

 of the low-pressure jet; and if of thick make, which can be 

 judged by the thickness at the edges, they are pretty durable 

 even under heavy weights. But the best of all are made of 

 sheet india-rubber, cemented between two fabrics. These are 

 the most expensive, but wear so long, that for anything like 

 hard wear they are far the most economical in the end. As 

 india-rubber ' perishes ' in time, however, for only occasional 

 use it may be better to use the cheaper black twill, and when 

 necessary procure another. 



When a bag leaks, so long as the leaks are circumscribed 

 and definite, it may be patched so as to last a considerable 

 time longer. At india-rubber shops, a shilling tin can be 

 purchased of ' india-rubber solution,' which is principally the 



