The Liverpool Standard and General Commercial Advertiser.
Liverpool, Merseyside, England, 1836-10-11 00:00:00

LECTURES UN OPTICS, MECHANICS' iNsTrru rtoN.

On Wednesday Evening, Robert Addams, Esq., of London,
celebrated for his lectures on natural philosophy, commenced a
course of ten lectures on optics. at the Mechanics' Institution, to
a crowded audience, which he illustrated with a !member of
beautiful and conclusive experiments. There are few lecturers
who possess such an admirable collection of apparatus as that
which Mr. Addams exhibits during his lectures.
Light, he observed, is obtained from the luminous bodies of
the heavens, and from the combustion of gaseous matter. In
the latter, its intensity is dependent upon certain portions of
solid matter contained in it. Thus coal gas is a compound of
carbon and hydrogen. Hydrogen is the most inflammable of al'
gassis, but it burns with so feeble a light that we can scarcely
see it ; but if we mix hydrogen with charcoal it will enter into
combination with it, and produce the intense light we nightly
observe in our streets. This intensity is owing to the ignition
of the carbonaceous portion of the gas. Spirits of wine burns
so feebly that we cannot discern the flame in the day time ; but
if we mix with it any solid body, it will give increased light.
This Mr. Addams satisfactorily demonstrated by sprinkling
iron filings upon the flame caused by a sponge saturated with
spirits of wine, which produced brilliant sparks of intense
white light.
Light may be also produced by motion—by rubbing one
body upon another, especially if we take two pieces of flint or
of lump sugar. If these be rebb .d together in a dark room,
they will produce a pure white light, but faintish in intensity.
That this was the case, Mr. Addams showed by rubbing two
pieces of quartz (which may be called transparent flint)
together; the light emitted was sufficieet to be readily seen in
the lecture-room. Now it might be asked, in the first instance,
how light is produced in this way ? N'Ve might say, in regard
to time sun, that there is a fluid continually emitted from it
which causes light ; but where is the light in the substances
we rub ? How is that light produced ? Is it excited by the
motion of rubbing, or does it exist in the mass of the body,
and, if so. how is it brought out ? If we suppose it owing to
motion, we shall then. by and bye, be able to connect it with a
theory affording are explanation of the phenomenon.
'Mere are certain minerals which, if heated to a less degree
than is requisite to make them red hot, will yield a pure and
white light, resembling that emitted by the glowworm. Fluo-
ride of calcium, or Derbyshire spar, and phosphate of lime,
are of this class ; Mr. Addams sprinkled some of their
dust upon a heated iron-plate. which confirmed what he had
advanced. After these particles have once emitted light in this
manner, they will not again yield it upon a repetition of the
experiment unless in the meantime a current of electricity be
passed through them. This can only be explained by supposing
that the particles undergo a change of shape during their heat-
ing, and that the vibration or motion attending this change of
shape produces the light; that no change of shape attends the
second heating, but that the electric shock, by restoring the
crystalline particles to their original figures again, put them in
a situation for undergoing a change on the application of heat.
The lecturer saw no other mode of accounting fur these facts,
unless by such a mode of reasoning.
We may ask then what is the cause of light. It is a ques-
tion very easily put but not very easily answered. it has been
commoa for persons to suppose that light is a form of matter,
that it is constituted of minute atoms of matter, which are
shot off from the sun, with the immense velocity of 192,000
miles in one second of time. Now it seems to us almost im-
possible to conceive that particles of matter so uncommonly
small, can be made to travel with such a velocity, although we
know nothing is impossible with the divine being. We know
that if we take a quantity of gunpowder and put it into a can-
non, with a quantity of dust, we shall find upon the discharge
of the powder, that the dust will be ejected a very short distance.
Thus, if we diminish the particles of matter, the less will be
its velocity. And how small must be the particles of light
when they do not injure our eyes or any other article upon
which they impinge with such great velocity. This notion is
opposed by ma theory which considers light to be produced like
sound, by a succession of waves set in !notion. It would appear
absurd to say that sound assumed a shape or form. There is
nothing setting out from the sun to our eyes, but simply waves
put in motion. But what is put in motion, the theory of undu-
lation supposes that all nature throughout the universe, as well
as the infinity of space, is filled with a very refined kind of
fluid, millions of tinies finer than the common air we breath.
This fluid is called (ether for want of a better term. Now this
matter possesses such extreme elasticity, that if it be struck in
any part, the tremor will pass through it at the t ate of 192,000
miles in a second, whilst sound only travels 1130 feet in time
same time, in our atmosphere of mean density. Notwithstanding
this inconceivable speed, it occupies Many years, nay centuries
in coining from some of the luminous bodies in the heavens to
our earth, and it is in reference to these bodies that we are able
to ascertain the velocity of light. One of the principal methods
by Whirls it is arrived at, is by observing the time at which the
satellites of Jupiter are obscured, and noting the length of time
durfng which they remain invisible to us. We find that at one
period of time year, in the earth's revolution round the sun, we
are much nearer to Jupiter than at another ; by ascertaining the
difference between the time of the obscuration of the satellites,
which takes place when the earth is farthest from, and
nearest to Jupiter, we find the time which light takes in travel.-
ling a distance equal to the two positions of the earth, at the
two periods stated, and find this to be 192,000 miles In a second.
This is only one of the proofs we have, Dr. Bradley describes
another connected with the phenomena of the aberration of
light.
Now, according to the undulatory theory, the sun strikes the
,eether as a bell strikes the air ; that is, the sun may be always
iii a state of vibration by heat, electric action, or other causes
by which the material domain is put in motion in every direction
.in a ;manner similar to the air when struck by a bell. This is
the most reasonable method of explaining the action of light.
From the phenomena of polarization and other proofs, we must
suppose time atoms tnoving transversely while they are moving
along, that is to say in alternate curves, like. the waves of the
ocean. The waves of light vary in rapidity and length with
their varied colours : red is the shortest and least rapid of the
colours, and there are 39,180 red waves in an inch, which tra-
vel at the rate of 477 millions of millions of miles its one
second. '!'lie violet wave travels fastest, proceeding at the rate
of 699 millions of millions of miles in a second.
The question now arises, which of the two theories should we
adopt. To say that the sun gives out tome, would argue that
it will be wasted in time. Let us see which of the theories will
account for the most of the properties of which we find light to
be possessed. The corpuscular theory is the older theory,
which was adopted by Newton and his contemporaries ; whereas
the unduletot y one is of comparatively modern standing.
There are, altogether, 23 orders of phenomena to be accounted
for in optics, such as time polarization of light, its motion, &c.
The corpuscular theory only explains four out of these perfectly ;
there are three imperfectly explained, one perfectly by a condi-
tion supposing that there is a repulsion taking place on the
surface of a plate of metal—three imperfectly, by assuming
coollitions—and twelve cannot be explained at all. By time un-
dulatory theory seventeen orders of these phenomena are ex-
plained perfectly, three imperfectly, one conditionally, there is
no order left explained imperfectly with conditions, and only
two which cannot he explained at all. Indeed these two have
lately been explained. After this, the lecturer said he need not
say which theory Ought to be adopted, that which explained
satisfactorily seventeen orders of phenomena, or that which
explained only four.
The intensity of a light is the quantity of illumination it will
produce. We often find it convenient to determine the differ-
ence of intensity between various lights. Suppose there existed
a prejudice against the use of gaslights in favour of that of
candles, it would be necessary to show time consumer that gas
gave a greater quantity of light at less expense. To do this we
must be able to compare the light of a candle with that of a
gas hum. This will lead us to consider what is the de-
crease of light according to the distance of the luminous body,
and then we shall get nearer a comparison. Now, suppos-
ing a.candle be placed at a given distance from a wall or piece
of paper, it wouldethew a certain quantity of light upon the
wall, but, if the candle be removed to a greater distance, it will
not illuminate the wall as much as it did before. In fact, we
find bra very simple experiment that the quantity of light is
inversely, as the square of the distance of the luminous body.
By this tneans we calculate the distances of the planets by no-
ticing- the -reflection.. whictr they cause at various distances.
This is precisely what happens with regard to sound. and gave
the idea of a photometer to Count Rumford. His method, es
well asthma of Dr. Black, and other celebrated experimentalists,
consisted .chiefly in intercepting time light from is candle and
lamp by means Of some opaque body, mid then' noticing the
difference of the shadows. l'he luminary occasioning the
strongest shadow was removed to a greater distance, until
the shadows cast by each light was exactly similar, then sup-
posihg one 'to be ten feet', and the other five feet from the inter-
cepting substance, these numbers. were squared, which spewed
diet elle light at the greater, distance gave out 100 parts of light
to 25 of the tither, endciiiiieqUently, that it was of fourtimes its
intensity, variousingenjone contrivances have been devised by
Dr. Ritchie and others, for facilitating this expeFiment, the
apparatus (or which shewil and-explained by the talented
lecturer.
'On Saturday evening fur. Aduanis continued,
'There eire'vatious-causeewhiele disturb the rectilinear motion
always in time direction of
'of a ray of et A ray of light is
the ratliUSOf a system of spiral waves, emanating in every pos-
sible direction from the luminous body, and would always pursue
a perfectly straight oeuree, provided the media through which
time waves arc propagated were of the mime density and elasti-
Beteif these waves or 'undulee meet with a medium of
;mother density they will assume anew direction. if the op-
pesing medium be metal, we shall have a new eystein of waves
propagated from its surface, passing off perhaps in a diagonal
direction. Light. ansy• be supposed, to be reflected from any
surface, even the air itself reflects a portion of light. We may
safely say there is not any body which is visible but reflects a
portitni of light, but the light bodies reflect snore than the dark,
and so on. The law of reflection is this, that if a ray of light
fall perpendicularly upon any reflecting surface, it will be re-
turned hack in the same direction, but if the ray make an angle
with the surface of reflection, it will be reflected on the opposite
side of a perpendicular, so as to make the angle of reflection
equal to that of incidence.
We must confine our observations to those bodies which ap-
pear lustrous, such as a piece of glass silvered at the back, or
else a piece of polished metal, because, if we bold a piece of
paper tip, we see the light the same in whatever way we hold it,
but riot so with silvered glass or polished mend, if we hold
them in one position We do not see the image of the light, hilt
in another we do. To this description of surfaces therefore, the
law previously given will out}, apply.
According to the undulatory theory, reflection is caused by
'a change in the direction of the- ray of light, in consequence of
the waves of light encountering a new medium of different
elasticity. The density of the tetherial fluid yam ies in different
bodies. It has before been observed that the space through
which the heavenly bodies pass, is a void filled with this
'file pores of our atmosphere mare likewise filled with this tether,
as well as Witter and every substance in stature. As the density
of the medium is increased, so is the elasticity and the velocity
of the waves of light diminished.
Having explained the cause of reflection, it remains to be
shown how light is only seen from it polished. mirror, in that
angle called the angle of reflection. Suppose two rays of light
to fall diagonally
upon an horieontal surface when they touch,
the plane there will be
a new system of waves generated, tint
a reflection of a poetioe of the light, not of all, for a portion
will pass into the opposing :mance, which will be greater or less
por-
tion to the Stibitance of which it is composed. That
of the rays which is reflected,
will move on
cession of circles. When two rays are coming in
d a own
to
mil
-
oblique direction, parallel to each other, it is necesssa.roy_i_p7s-8
vent the waves interfering with each other-, that theY,rnhous.64en
over equal awes at the same period of
calshort space to
one ray has reached the surface, the other :silt:
e me time first
travel through before it reaches it, flaring w ratiteas arrived
ray is passing off at an bogie. Whet' the becony
at the surface, the first has. passed, brougli a short space, and
other. This law
the two rays again proceed parallel to each
fdi ris
difficult to be understood me with the aid o ag ams, but the
reader will tee that as it is necessary that c (I,mil spaces should
be described by each ray, that time angle of lucidence will always
Were there any differ-
be the same as the angle of reflection.
ence in the spaces described
by the different rays, one ray would
destroy the other.
The quantity of light reflected by different bodies, is exceed-
ingly variable, Some will reflect ,
one half, others not more