USES OF THE MAGNESIUM LIGHT. 133 



made of gutta-percha, or fitted with gutta-percha surfaces, 

 which substance holds it with a sufficiently firm grasp to 

 draw it forward between them as the wheels revolve. G 

 is the key by which the clock-work is wound up when it 

 runs down, and at T is the tongue of a little catch by 

 means of which the clock-work may be set going or 

 stopped at pleasure. The wire of magnesium is burned 

 at the end C, which protrudes in front of the concave mir- 

 ror, being pushed forward by the clock-work as fast as it 

 burns, while the magnesia that results from the combus- 

 tion falls down into the pan E below. F is a thumb-screw 

 connected with rack -work, by which the mirror can be 

 moved backward or forward as required. The whole can 

 be taken up by the handle, which serves, when the lamp is 

 stationary, as one of the legs. 



The magnesium light is used chiefly as a substitute for 

 the light of the sun in photography, especially in cases 

 where the light of the sun is not at command, as, for in- 

 stance, in caverns, and mines, and other dark places. Its 

 intensity, and certain chemical properties which result 

 from, or, at least, accompany this intensity, fit it to answer 

 these purposes extremely well. 



It has been used in this way very successfully in photo- 

 graphing interior views of the great pyramid in Egypt, and 

 in many other similar cases, where none but artificial light 

 could possibly be obtained. It is also sometimes used for 

 engineering and military purposes, such as for illuminating 

 works of construction when it becomes necessary to carry 

 them on at night, and also for showing the position and 

 movements of the enemy in case of nocturnal operations in 

 war. When, for example, the garrison of a besieged city 

 wish to make a sortie at night, if they can send off in ad- 

 vance, or at a little distance from them on one side, an in- 

 tensely brilliant light, their enterprise is greatly aided, and 



