SOUECES OF ILLUMINATION 87 



rotate the lime while the jet is burning an adjunct which is 

 indispensable with all forms of jet. Fig. 24 is a type of mixed jet 

 for use with the two gases underpressure. In this one a special 

 cut-off (devised by Mr. Andrew Pringle) is fitted, so that the 

 light can be turned up or down without altering the relative 

 adjustment of the taps. This is a great convenience in photo- 

 micrography as, usually, the exposures are short, and the light is 

 only required for short periods. 



The Gwyer jet is another type for use with mixed gases, and 

 is the most powerful at present to be obtained. In its smallest 

 size it is stated to give a light of approximately 1500 candle- 

 power, and the larger sizes considerably exceed this. The 



FIG. 25. Adjustable Lime-light Tray. (Gwyer Jet Co.) 



standard patterns are provided with a tap to cut off both the 

 gases, and also with screw valves for exactly regulating the 

 supply of gas to each tube. With the cut-off tap the gases are 

 reduced in succession, and only a small hydrogen by-pass flame 

 is left. The position of the screw valves is not altered, so that 

 the light can be obtained at once in its original condition 

 without re-adjustment of the relative quantities of the gases. 



It is convenient to support any jet on a centreing- tray. 

 This is an arrangement by which lateral or vertical adjustment 

 can be obtained by the rotation of a milled head controlled 

 from the back. It is so essential in microscopic work for the 

 light to be accurately centred, that this is almost a necessity. 

 The mechanical stand shown in Fig. 24 is a good type, and the 

 light may be centred without fear of altering its position. Any 

 such appliance should be well and strongly made, as otherwise, 



