Appendix II 167 



For a similar reason the quadruple acetylene jet is not to be 

 recommended. As a rule, the four separate jets will produce four 

 separate images on the screen, and if acetylene must be used, it is 

 better to use a single high-power jet of the duplex type than two 

 or more separate jets. 



The best radiant is thus one of high candle-power upwards of 

 500 must be available for apparatus projection and erection and 

 small area, and the appliances which most nearly approach to this 

 ideal are 



(1) The electric arc. 



(2) The Nernst lamp (star pattern). 



(3) The limelight, various patterns. 



(1) The electric arc is, as a rule, obtained by an appliance 

 specially built for lantern work, the best examples perhaps being 

 the " Brockie-Pell," " Ross-Hepworth," or the " Phoenix " lamps, 

 though, of course, there are others. These lamps have the great 

 advantage of fitting standard lanterns. The electric arc may be 

 worked at any pressure over fifty volts, and may be controlled by a 

 number of resistance coils, or, if the alternating current be used, 

 by a choking coil. The carbons of the lamp should be sloped 

 at an angle of 60 or 70 to the horizontal, and the current should 

 pass down the lamp, i.e. the upper carbon should be connected to 

 the positive wire. With the alternating current, the carbons are 

 best arranged horizontally. By this means the arc is placed 

 so that the crater of the upper carbon throws most light outwards 

 towards the condenser, in an equally illuminated divergent cone. 



(2) Failing the electric arc, " Nernst " lamps may be used, but 

 these are delicate, and have the serious objection of requiring a 

 considerable time to light up. They are not recommended for 

 general use, especially if, as is usual, the lantern is required 

 intermittently during a lecture. 



(3) The next best light is the limelight, and many variations 

 of this are quite common. This illuminant is the most popular of 

 all, being safe, clean, easy to handle, and of sufficient power for all 

 ordinary work. It has the great advantage of portability, and 

 is, of course, independent of an electric installation. 



Thus we have the simplest kind, the oxy-coal gas flame, used 

 with an ordinary blowpipe (" blow-through ") jet, the oxygen being 

 obtained from a cylinder of compressed gas ; the oxy-hydrogen 

 flame, both gases being obtained from cylinders, and being used 

 either through a " blow-through " or a " mixing " jet ; the oxy-ether 

 flame, oxygen being obtained as before, and passed through a 

 " saturator " containing ether in order to give the flame, pure 



