ii8 OPTICAL PROJECTION 



additional tap is interposed between each of the usual ones and 

 the mixing chamber. These taps are geared together by cogged 

 wheels on the ends of the plugs, so that both are turned simul- 

 taneously by a bevelled cog-wheel worked by a spindle from 

 the back of the jet. The plugs are so fitted that when the 

 key at the back is turned as far as it will go in one direction, 

 the jet is full on so far as these taps are concerned ; and when 

 turned the other way the oxygen is cut entirely off, and the 

 hydrogen all except a bye-pass. When full on, the light is 

 adjusted precisely as usual by the usual jet-taps ; therefore, 

 when turned off and on again, this adjustment is found pre- 



FlG. 65. Jet with Pringle's Cut-off 



cisely as it was left. With a lantern thus fitted at an extra 

 cost of about 15s. per jet, such an intermission occasions no 

 difficulty whatever. 



Another ' cut-off ' jet has been constructed by Mr. Oakley. 

 Each plug of the ordinary taps has a screw piercing down 

 the centre from the top, into the gas-way. The gases are 

 adjusted by screwing these down so as to obstruct or free the 

 passage, while each tap can be turned entirely off at pleasure, 

 the hydrogen tap having a small channel all round as a bye- 

 pass. The two taps have, however, to be turned separately, 

 and some care is requisite to keep the adjusting screws gas- 

 tight. 



64. Tri-unial Lanterns. By adding a third lantern to 

 the top of a bi-unial, we get the tri-unial lantern, as in fig. 66, 



