OPTICAL TELEGRAPHY. 
13 
made by the French Telegraph Administration in 1870, 
enabled communications to be made between Havre and 
Honfleur, when the submarine cable was broken, and the like 
has occurred at other places, especially between Paris and the 
detached forts. The Prussians also used red and green signals 
during the siege of Belfort. The committee for organizing 
the national defence of Marseilles proposed in November, 1870, 
to the government of Tours, a system of night signals founded 
on the emission of rays of light for a longer or shorter period, 
so that the Morse code might be used. This proposal was 
considered at Tours by a specially-appointed committee. The 
object of this scheme was to communicate with Paris from 
without, over the first Prussian line of investment. This line 
had at that time a radius not greater than twenty-seven miles, 
and the proposed communication might have been effected had 
prompt measures been taken. The great extent that was 
given to the second line of investment caused the inventor to 
abandon his project. In the meantime, M. Lissajous who had 
left Paris in a balloon with a similar project, showed the 
provinces how easy it would have been to establish a system 
of communication. He got M. Santi, the skilful Marseilles 
optician, to construct apparatus founded on the same principles, 
but these apparatus did not come into use, or at least not 
during the war. They have, however, been taken up since by 
the military Telegraph department and are now used in our 
army. 
The military telegraph administration has daily experiments 
made at the military school at Saumur, and once a year experi¬ 
ments on the large scale are made at the camp at Saint-Maur. 
The apparatus used at the present time is that proposed by 
Colonel Mangin, and it may be thus described. A rectangular 
box, a, Fig. 3, is divided into two equal parts by the 
diaphragm, b, which has a very small round opening at c. 
A convex lens is mounted in the front part of the box. This 
lens has a diameter of 5-|, 9^, or 13f inches, as the case may 
be. The first two diameters are those most in use. In front 
of the opening, c, is placed a screen, d, capable of turning on 
an axis in such a manner as to open and shut the orifice by 
means of a handle at m. A touch of the finger suffices to raise 
this screen, and it falls back into its place by its own weight. 
