64 OPTICAL PROJECTION 



If the empty chamber will work quietly, it is nearly as good. 

 Either is a true safety arrangement, because if by any careless- 

 ness an explosive mixture did occur, it would occur first in the 

 chamber, which would * snap ' the jet out at once. 



One other pattern of jet deserves mention, i.e. the 

 ' Injector ' jet. This is essentially a mixed-gas jet, the coal 

 gas being supplied from the main. With care, any ordinary 

 mixed jet can be used in this way, and the oxygen being 

 supplied from a compressed cylinder with a good regulator, 

 such a method of working is perfectly safe. Without very 

 great care in adjusting the oxygen tap, however, the jet is 

 liable to snap out, and in any case the light obtained is very 

 little if at all better than with the blow-through form. 



With the ' Injector ' jet, however, the oxygen blast sucks 

 more coal gas out of the mains, and thereby the necessary 

 pressure is obtained. The greater the pressure of oxygen 

 used, the more coal gas is drawn out to combine with it, and 

 hence it is hardly possible to ' snap ' these jets. 



The light comes about midway between that given by the 

 - mixed gas and by the blow-through forms. It is sometimes 

 advertised as being equal in power to the mixed, but I have 

 never found it so in practice. 



To get the best light a higher pressure of oxygen is re- 

 quired than is given by the usual pattern Beard's Regulator, and 

 Mr. Beard constructs a special regulator for use with this jet. 

 The best lime-turning movement, in my opinion, for prac- 

 tical work, of all that have come under my notice, is that 

 shown in fig. 43, which was originally devised by Mr. J. 

 Place. The long steel spindle has a longitudinal groove 

 cut, which slides up or down over a feather in the cog-wheel ; 

 and there is also a long-pitched spiral groove which gives 

 the required motion. To this Mr. Newton has added a 

 small wheel on the manual rod, in which notches are cut at 

 proper intervals. A spring detent bears against these notches, 

 so that the exact spot at which the line should be held is 



