APPLICATIONS OF THE ELECTRIC LIGHT. 273 



worked by four men, gives a light equal to 50 Carcel lamps. 

 The French Government has lately had it tried. 



Experiments with machines arranged in nearly the same 

 way were made in Berlin in 1875. The light produced was 

 intense enough at a mile's distance to allow ordinary writing 

 to be read. When a mirror placed in front of the regulator 

 was inclined so as to reflect the rays upwards, a luminous 

 track was thrown upon the clouds, and from a distance 

 appeared like the tail of a comet, in which the signals made 

 before the^ mirror showed themselves. 



Mangin's projector is the one adopted in France for mili- 

 tary operations, and it is in arrangement somewhat similar 

 to that of Siemens', which we have shown in Fig. 70. It is 

 mounted on a low light truck, by which it can easily be taken 

 wherever required. This apparatus, described in detail in 

 the Memorial de I'officier du genie (Engineer Officer's note- 

 book), is composed essentially of a concavo-convex glass 

 mirror with spherical surfaces of different radii. The con- 

 vex face is covered with silver, and reflects. This mirror, 

 go centimetres in diameter, has the property of being free 

 from spherical aberration, notwithstanding its diameter and 

 its focal length being nearly equal. 



Between the mirror and the luminous arc is a concavo- 

 convex lens with its concavity towards the light. Its use is 

 to collect upon the mirror a greater number of the rays, and 

 thus increase the amplitude of the field of illumination. 



The beam leaving the apparatus when the lamp is at the 

 focus of the lens is exactly bounded by a circumference 

 almost free from penumbra, and without other divergence 

 than that due to the dimensions of the source of light, which 

 divergence is about 2\ degrees. The light is uniformly dis- 

 tributed over the whole surface. 



This apparatus also possesses the property of being made 

 at will, either to light up a considerable space or to concen- 

 trate its intensity on one point, and this renders it extremely 

 suitable for certain military operations. A simple displace- 



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