1823.] 
the coloured rings that light produces, 
in passing through the laminz of crys- 
tals, ihe colours that polarized light de- 
velopes,, in passing through those la- 
minz, become thus evident and neces- 
sary consequences of one and the same 
theory... 
Wih:en.two rays, issuing from a com- 
mon source, mect. on the same point of 
a surface, the double effecis of light are 
not always in force, but may destroy 
each other. Aud so the union of two 
luminous rays may produce. obscurity, 
an_ effect which takes place in scyeral 
experiments. In results of this kind 
the principle of interferences consists, 
which may be considered as the most 
fertile and extensive in this new part of 
optics... The origin of it may be traced 
tv the experiments of Grimaldi, which 
were a precursor to, Newton’s Optics, 
also to the Researches of Hook; but very 
much is owing io Dr. Thomas Young, 
who bas intreduced it, with demonstra- 
tive proofs,.into the study of tle pheno- 
mena of optics. 
It. should be observed, that this prin- 
ciple, is not, exclusively, couliued to 
optical properties. M. Arago has shown, 
that, when the meeting of two trays 
causes their annihilation, the chemical 
action.of light disappears likewise. 
The design of M. Fresnel, in his most 
recent researches, is to designate the 
mathematical laws of double refraction 
in all crystals, together with the quantity 
of light reflected by diaphanous bodies 
in different peints of incidence, and also 
a kind of polarization very different from 
that hiiherto noticed, but which pos- 
sesses characters as general and as con- 
stant, 
A practical illustration of some of the 
properties of light appears in the esta- 
blishment of dioptric pharoses, or light- 
houses. In these, the light is not. re- 
flected, but tausimitted through glass 
lenses, which render the rays parallel. 
The flame is placed in the centre of 
cightsimilar lenses, and the whole turns 
on an axis, so that all the points of the 
horizon are. illuminated. ‘The light is, 
alternately, more and less ardent, diver- 
sifying and distinguishing the points of 
flame, M. Fresnel has formed lenses of 
large dimensions, consisting of scveral 
parts; in (these, be does away all the 
thick and: heavy, parts, which only tend 
to weaken tle Jight, a disposition not 
unobserved by, Buffon. 
To render the flame, uncommonly 
ardent, Messrs. Arageaud Lresnel have 
inventcd w lamp with concentric fires, 
Institute of France. 
443 
the light of whieh is equivalent to that 
of 150. bougies. 'rom late trials, it 
appears that even in dusky weather, 
these lights may be seen at the distance 
of more than eight leagues. Such is 
heir lustre, that even before the close of 
day they may serve as signals in geodc~_ 
sic operations, and haye been employed 
as such by Messrs. Arago and Mathieu, 
and by Messrs. Kater and Colby, of the 
Royal Socicty of London. A telescope. 
will discover these signals at more than 
sixteen leagues distance, an hour before 
sunset; and, an hour after sunset, the 
naked eye will distinguish them at the 
same distance. 
The discoveries that have been made, 
of ate, in the theories of electricity and 
magnetism, take their rise from the no- 
table experiments of M. Oersted, of the 
Academy of Copenhagen. Long conti- 
nued trials and speculations on the 
identity of the causes of electricity and 
magnetism, led him to observe that the 
conducting wire which joins the two ex- 
tremities of the voltaic apparatus, has a 
very seusible influence on the direction 
of the magnetic needle, and he detailed 
all the general characters of this, pheno- 
menon. ‘The Academy of Sciences of 
Paris decreed one of its, great annual 
prizes to M. Oersted, concluding that 
ibis discovery would lead to others, and 
perhaps to a physical and mathematical 
theory ; the event has been conformable 
to this expectation. 
M. Arago was the first to observe a 
remarkable fact connected. with the 
Danish process, that the same conductor 
which transmits the electrical current, 
aitracts iron, and communicates to it the 
properties of the loadstone; and that this 
effect ceases as soon as the current is in- 
terrupted. 
M. Ampere has been pursuing bis 
enquiries respecting the general laws of 
the dynamic actions of the conductor 
and magnets. He finds that a, mutual 
action, attractive and repulsive, cxists 
between the conductors, subject to cer- 
{ain conditions; a curions, discovery, 
from which he has deduced a great num- 
ber of facts. As to the action of magne~ 
tized bodies, M. Ampere attributes it to 
the presence of a multitude of electrical 
cirenits, formed, about each molecule, of 
such bodies. . If the existence of, these 
currents cannot, be positively, asserted, 
it is, at least, evident that the magnetic 
properties are reproduced, very sensibly, 
when we. give. to, the conductor the 
figure of a helix, the spirals of which 
are considerably multiplied, This shows 
what 
