76 
POPULAR SCIENCE REVIEW. 
source of economy where numerous copies of the same 
despatch are required to he delivered at the same time — as, 
for example, at various newspaper offices. The various letters 
are, however, sent as rapidly by Henley’s more simple machine, 
which prints by dots and dashes. Probably, Hughes’s American 
printing telegraph, in which the letters are printed in Roman 
type at once, will eventually come into general use. 
Of all the numerous trophies in the nave of the Exhibition, 
the liolophotal light-house revolving apparatus of Messrs. 
Chance is the most dazzling and one regrets that there is not 
an opportunity of witnessing- the flashes of light which it would 
send forth were the building in a state of darkness, and the 
great lamp at its centre duly burning. How different from the 
first attempts at light-houses — where a fire of flaming coal or 
wood warned the mariner of his approaching danger, or even 
from the feeble light of a few candles, without either metal- 
reflector or “ bull’s-eye,” which for half a century surmounted 
Smeaton’s noble structure. Each of those curved and annular 
prisms and lenses are ground and polished with mathematical 
exactness in order to reflect and refract, and so economize 
every ray of light proceeding from the four eoncentric-lighted 
wicks, and send them far and wide round the horizon. With 
one of those massive glass cages the light may be seen at a 
distance of thirty miles. The dioptric light-house apparatus, 
although often thought of, has only been successfully realized 
within comparatively few years, and that mainly through the 
exertions of Fresnel. The combination of metal reflectors and 
lenses proposed by that eminent philosopher, who busied 
himself as much with the humble improvement of the lamp 
itself as with the deeper researches on the form and arrange- 
ment of the lenses, is, however, here much simplified, and 
glass is made to perform the whole duty. The perfection to 
which the manufacture of this material has been carried, and 
the improved plans introduced by Mr. Stephenson in the 
arrangement of the prisms by which the divergent rays of the 
flame are gathered up and distributed in one uniform horizontal 
beam, have fully proved the excellence of the change. The 
lantern revolves on its axis by means of clockwork machinery, 
which keeps an uniform rate of motion, and by the number of 
flashes, emitted the sailor is able to tell the name of the par- 
ticular lighthouse to which he is near. Among other articles 
exhibited by this firm, we might point attention to the discs of 
glass of twenty-five inches in diameter, prepared for optical 
purposes, and from which it is to be hoped that, if perfect, they 
may speedily be worked up into the object-glass of a large 
refractor. 
When magnetic observatories were first established, it was 
