i6 
THE TELEGRAPH. 
The same apparatus is used in the daytime, but the lamp is 
then removed, and its place is occupied by a lens for the 
purpose of condensing the solar rays to the position of the 
flame of the lamp. In the apparatus with a lens of 91- 
inches diameter, two small plane mirrors, which are adjusted 
by hand, throw the solar beam in the proper direction. The 
diurnal motion of course requires the mirrors to be adjusted 
from time to time, but this is easily done by hand every five 
minutes. In the larger apparatus a heliostat is fixed on the 
upper part of the box, and is provided with clockwork by 
which the mirror is made to follow the apparent motion of the 
sun, so as to direct the beam always in the same direction. 
In cloudy weather the night apparatus can be used for signal¬ 
ling in the daytime, as a petroleum lamp is able to give 
perceptible signals even at a distance of 12 miles. 
Experiments have been made with mauy apparatus at 
the camp of Saint-Maur. Amongst the rest was one with 
polarized light, in which the signals were produced by the 
polarization of the beam when a Nichol’s prism was intro¬ 
duced into its course. Thus, the beam of light being uninter¬ 
rupted ail the time, the signals would not be perceived by 
strangers. 
An Italian system, with red and green lights, has also been 
successfully tried. In this case also the beam of light con¬ 
tinues without eclipse, and is merely tinted by the intro¬ 
duction in its path of red or green glasses which the operator 
holds in his hand, and uses as easily as a pair of drumsticks. 
M. Mercadier has produced an apparatus in which the com¬ 
bustion of the lamp is quickened by a jet of oxygen gas. An 
apparatus of this kind invented by Mr. Walker has for some 
time been made in England : it is manufactured at Silvertown, 
and it includes even the apparatus required for making oxygen 
gas in the field. 
In all these systems the speed of transmission amounts to 
twelve or fifteen words per minute, and by experienced 
operators it may be increased to twenty words per minute. 
It was feared that the persistence of the impressions of the 
signals upon the retina would necessitate slow transmission. 
But it has been found in practice that there are no grounds 
for that supposition. The running together of signals that 
