424 



OPTICAL PROJECTION 



aforesaid, the evaporation does not decrease nearly so much as 

 when no reservoir is provided, only the cooling effect having to be 

 reckoned with. 



(3) The operator knows that so long as he has any free ether 

 left in the lower tank his saturator is full a point very difficult to 

 be certain of with most saturators. 



Mr. J. Hay Taylor, writing in the * Optical Magic Lantern 

 Journal,' reports that he obtained an excellent and steady light for 

 three hours with this saturator, using a mixed gas jet of large bore ; 

 with a smaller bore the supply will, of course, last even longer. 



Nernst Electric Lamps. These lamps have now come into 

 general use. Where a moderate supply of electric current only is 

 available, and the greatest perfection of light and definition is not 



required, they are an ex- 

 tremely convenient and 

 simple illuminant. They 

 are usually mounted to fit 

 on to an ordinary lime- 

 light tray (see fig. 239), 

 either one or three fila- 

 ments being employed, 

 and the resistance (always 

 supplied with the lamp) 

 enclosed in a glass bulb 

 behind. 



Each of these fila- 

 ments carries roughly one ampere on a 200- volt current, and by using 

 a lamp with three of them as much as 1,000-candle power can be 

 obtained, but owing to the length of these filaments the effective light, 

 as well as the definition, are by no means equal to the electric arc. 

 One peculiarity of these lamps is that no current can be passed 

 through them until the filaments are heated. This can be done by 

 means of an automatic arrangement, but for lantern work it is 

 usually done by hand, holding a spirit lamp (specially supplied for 

 the purpose) under the lamp for a few moments. 



Once the current is started the spirit lamp may be removed, as 

 the filaments, of course, keep hot until the current is switched off. 

 If this is done for any reason, the spirit lamp must again be applied 

 before current can pass through the filaments again. 



Except, however, for the trouble entailed by this initial starting, 



FIG. 239. Nernst Lamp. 



