FLASHLIGHTS 343 
of magnesium ribbon, 3mm broad and 0.5 meters long, 
wound into a spiral torch which gives an exposure of from 
1%4 to 3 minutes; from three to six of these spirals will give 
exposures from 15 to 20 minutes; up to 10 meters one spiral 
is enough; up to 15 meters three or six should be used. 
Placing magnesium powder in 5 g lots and firing with a tuft 
of pyroxyline, one charge will be enough up to 10 meters; 
for greater distances successive charges should be fired. 
Flash powders are not satisfactory; they are dangerous and 
give more smoke. For longer distances blow-through lamps 
should be used, using about 8g up to 60 meters; up to 40 
meters 5 g. 
DuRATION OF FLaAsH.—Londe states that the duration of 
a flash is dependent on various factors; but, calling the 
unknown ingredients of a powder X, the following table 
gives the duration: 1. X 100, magnesium 150, duration of 
flash, 0.07 seconds; 2. X 100, magnesium 175, duration of 
flash, 0.10 seconds; 3. X 100, magnesium 225, duration of 
flash, 0.11 seconds; 4. X 100, magnesium 250, duration of 
flash, 0.12 seconds. 
Old powder fires more slowly than freshly mixed. The 
weight of the charge plays a part in the duration: 1 g burns 
for 0.03 seconds, 2g for 0.05 seconds, and 3¢ for 0.07 
seconds. 
The particular manner in which the powder is laid also 
affects the flash: 1 g in a heap burns for 0.2 seconds, while 1 g 
spread out burns for 0.4 seconds. 
The method of firing also affects the duration: Electric, 
duration 0.14 seconds; percussion cap, 0.144 seconds; match, 
0.188 seconds; touch paper, 0.280 seconds. : 
Any flash longer than 0.12 seconds will show movement 
of the eyes. 
