82 
PRACTICAL PHOTO-MICROGRAPHY. 
time that the light is required. To the mixing jet is 
fitted a device known as a “ cut-off,” which consists 
essentially of two extra taps, one for the hydrogen being 
a partial stop, the other for the oxygen being a complete 
stop, on the gas-way. 
The jets are so made that the light is arranged at its 
best point with the ordinary taps, the cut-off taps, both 
worked by one hand-piece, being wide open; when now 
the extra, or cut-off, taps are turned down the oxygen is 
completely turned off, while a small jet of hydrogen 
continues to burn at the nipple. When the cut-off taps 
are again turned on the light returns to its original 
point ol brilliance, and so gas is saved, and we have a 
constant light. Without this device we should either 
have to let the light burn during the time we are 
Fig. 26. 
developing a plate, or we should have to guess at the 
same power of light used for the previous exposures. 
The cut-off arrangement was originally made by Messrs. 
Newton, of London, to the writer’s design ; but can be 
obtained in other forms from several opticians, Mr. 
Baker among the number (fig. 26). 
In lighting a lime-light burner it is well to turn on 
first a little hydrogen and light it; then to work up the 
