This is the theory of undulation, as propounded by its inventor or discoverer; and it has been supported by the researches of Descartes, Hooke, Euler, Young, Fresnal, and others equally great of the present day. As the science of optics advanced, however, it was found necessary to modify some of the particulars, and to enlarge and add to the hypothesis, in order to explain the numerous and complicated phenomena which have been discovered since the time of Huygens. The principles on which the theory was founded did not originate entirely in the imagination, but were supported by arguments and data of the most weighty kind. For instance, that a universally diffused medium, resembling that which has been denominated ether, does actually exist, appears to be undeniably proved by the phenomena of electricity; and the rapid transmission of the electric shock would lead us to infer that the electric medium was possessed of an elasticity as great as is necessary to be supposed for the propagation of light. In the present state of our knowledge, it is impossible to form an opinion whether this luminous ether, and the electric ether, are the same, even if we suppose that they both exist; but probably this will be determined eventually by experiment. The rapid progress which is now being made in the sciences of electricity, galvanism, and magnetism, and the undeniable proofs which exist of the identity of fluids which were heretofore considered distinct and separate, would incline us to believe that the same principles will, some time or other, be applied to the elucidation of the laws of heat, light, electricity and sound.

The following quotation from "Newton's Optics" contains some of that great philosopher's opinions on the subject.-"Is not the heat of the warm room conveyed through the vacuum by the vibrations of a much subtiler medium than air? And is not this medium the same with that medium by which light is reflected and refracted, and by whose vibrations light communicates heat to bodies, and is put into fits of easy reflection and easy transmission? And do not the vibrations of this medium in hot bodies contribute to the intenseness and duration of their heat? And do not hot bodies communicate their heat to contiguous cold ones by the vibrations of this medium, propagated from them into the cold ones? And is not this medium exceedingly more rare and subtile than the air, and exceedingly more elastic and active? And doth it not readily pervade all bodies? And is it not, by its elastic force, expanded through all the heavens? May not planets and comets, and all gross bodies, perform their motions in this etherial medium? And may not its resistance be so small as to be inconsiderable? For instance, if this ether (for so I will call it), should be supposed 700,000 times more elastic than our air, and above 700,000 times more rare, its resistance would be about 600,000,000 times less than that of water; and so small a resistance would scarce make any sensible alteration in the motion of the planets in ten thousand years. If any one would ask how a medium can be so rare, let him tell me,-how an electric body can by friction emit an exhalation so rare and subtile, and yet so potent? And how the effluvia

of a magnet can pass through a plate of glass, without resistance, and yet turn a magnetic needle beyond the glass?"

It may appear surprising to those who have not studied the subject, that light should be propagated to such an immense distance without the velocity being greatly diminished: but this is a point which is especially favorable to the doctrine of undulations. For if light consisted of particles of matter projected from a luminous body, the momentum would decrease in a ratio to their distance from that body; whereas the velocity of light is the same whether these wonderful molecules are produced from the striking of a flint and steel together, a slight transmission of electricity, the white heat of a wind furnace, or by the intense heat of the sun itself.

By the Huygenian theory, the impetus is first given by the luminous body to the molecules of ether in its immediate vicinity, from whence it is propagated by undulations; and it is a well established fact that all impulses are propagated in a homogeneous elastic medium with an equable velocity. The propagation of motion through an elastic medium is a subject of considerable obscurity, and is probably the cause of the difficulty of explaining complicated phenomena. But, as is observed by Dr. Thomas Young in the Philosophical Transactions for the year 1802, "if the impulse be so great as materially to disturb the density of this medium, it will be no longer homogeneous; but as far as concerns our senses, the quantity of motion may be considered as infinitely small. It is possible, that the actual velocity of the particles of the luminiferous ether may bear a much less proportion to the velocity of the undulations than in sound; for light may be excited by the motion of a body moving at the rate of only one mile in the time that light moves a hundred millions."

It would far exceed the limits of a paper of this kind, to enter into all the reasoning which has been advanced in support of the different positions of this theory. Enough has been said, we should presume, to suggest to the general reader (for whose service it is designed), an idea of its nature. As we proceed with the subject, further details will be mentioned; and then they will most probably be much better understood when coupled with the phenomena which they are intended to explain. The chief supporters do not believe the undulatory theory to be perfect-probably no one actually believes in it in all its particulars. Theory must, in this instance, be regarded merely as the expression of a general law. By the exertions of the later philosophers it has been so much improved however, and, as remarked by Herschel in his " Discourse on Natural Philosophy," "is so happy in its adaptation to facts, and in the coincidence with experience of results deduced from it by the most intricate analysis, that it is difficult to conceive it unfounded. If it be so, it is at least the most curiously artificial system that science has yet witnessed; and whether it be so or not, so long as it serves to group together in one comprehensive point of view a mass of facts almost infinite in number and variety, to reason from one to another, and to establish analogies and relations between them; on whatever hypothesis it may be founded, or

whatever arbitrary assumptions it may make respecting structures and modes of action, it can never be regarded as other than a most real and important accession to our knowledge."

It cannot be denied that one or two objections to the undulatory theory exist, which certainly appear formidable, but which it will be as well to mention before we proceed farther. In the propagation of motion through an elastic medium, one of the principal laws is, that if the elastic medium be uniform and homogeneous, all motions of whatever kind are propagated through it in all directions with one and the same uniform velocity, a velocity depending solely on the elasticity of the medium as compared with its inertia, and bearing no relation to the greatness or smallness, regularity or irregularity of the original disturbance. Thus, while the intensity of light, like that of sound, diminishes as the distance from its origin increases, its velocity remains invariable; and thus too, as sound of every pitch, so light of every colour travels with one and the same velocity, either in vacuo, or in a homogeneous medium. Now it is known that the deviation of light by refraction is a consequence of the difference of its velocities within and without the refracting medium, and that when these velocities are given, the amount of deviation is also given. From this it would appear to follow unavoidably, that rays of all colours must be in all cases equally refracted: and that therefore there could exist no such phenomena as dispersion or the separation of homogeneous light into the coloured spectrum. This is the best-founded objection which has been urged; and as yet the difficulty has not been overcome, although many attempts have been made.

Another objection was brought forward by Newton, and was considered by him conclusive against the doctrine, but has since been almost entirely overcome. If, it is said, there be a perfect analogy between light and sound, how is it that shadows exist? Sounds are propagated freely round a corner; how is it that light does not do so? A vibration propagated from a centre in an elastic medium, and intercepted by an immoveable obstacle having a small orifice, ought to spread itself from this orifice beyond the screen as from a new centre, and fill the space beyond with undulations propagated from it in every direction. This orifice ought to be seen in all directions as a new luminary, as in acoustics it is heard as a new source of sound. Many arguments against this objection are deducible from the laws of sound itself; for upon a strict examination it will be found, that sounds are not propagated round a corner with the same intensity as in their original direction. Any one may convince himself of this, by observing the sound of a carriage in the act of turning the corner of a street, or by holding up a tuning-fork to the ear and interposing now and then a slip of card. Dr. Young has satisfactorily shown that light is propagated round a corner, in explaining his third proposition of the theory of undulations, viz. "A portion of a spherical undulation, admitted through an aperture into a quiescent medium, will proceed to be farther propagated rectilinearly in concentric superficies, terminated laterally by weak and irregular portions of newly diverging undulations. For, at the instant of admission, the circumference of

each of the undulations may be supposed to generate a partial undulation, filling up the nascent angle between the radii and the surface terminating the medium; but no sensible addition will be made to its strength by a divergence of motion from any other parts of the undulation, for want of a coincidence of time. If indeed the aperture bear but a small proportion to the breadth of an undulation, the newly generated undulation may nearly absorb the whole force of the portion admitted: and this is the case considered by Newton. But no experiment can be made under these circumstances with light, on account of the minuteness of its undulations, and the interference of inflection; and yet some faint radiations do actually diverge beyond any probable limits of inflection, rendering the margin of the aperture distinctly visible in all directions: these are attributed by Newton to some unknown cause, distinct from inflection, and they fully answer the description of this proposition."

Now that we have stated the principal objections to the undulatory theory, and pointed out those phenomena which either cannot, or can be but imperfectly explained by it, we will proceed to show those which can. With this intention, and in order to render the subject, which is confessedly abstruse and intricate, as simple and intelligible as possible, we will commence with the well-known laws of reflection and refraction, but avoiding, as far as may be, the more complicated mathematical reasoning.

The sources of light are chiefly the sun, and combustible bodies in a state of ignition. In order to understand the way in which it is propagated, let us consider, for example, the flame of a lamp or candle. Every particle of it propagates around itself, as a centre, a series of concentric waves, which circulate in every direction, and intersect each other, but do not interfere. It is not supposed that in these undulations, the particles of the ether themselves though in continual motion are themselves carried along, but that they receive an impulse and transmit it necessarily to those with which they come in contact, in the same way as undulations are propagated in other elastic media, as, for instance, the waves of sound in the air, and those produced by a stone on the water. But in addition to the motion communicated by the ethereal molecule to the particles next in contact with it in a straight line drawn through the luminous source, a portion of the motion is necessarily communicated to all the other particles which touch it, and oppose its motion. Wherefore it is evident, that a new spherical undulation will be created round each particle of which it is itself the centre. But each of these secondary waves must be infinitely weak when compared with the original wave, as all the others contribute to its composition by that part of their surface which is the most remote from the luminous centre.

By a similar kind of reasoning, it will be evident why light, considered as the result of undulations, is propagated only in straight lines, always considering that the waves are progressing through a homogeneous medium. Each part of a wave propagates itself in such a manner, that the extremities are always comprehended between the same straight lines drawn from the luminous

centre. For though the secondary waves, produced by the contact of the primary one, also spread themselves out of this space, yet they do not concur to compose together at the same instant, a wave which terminates the motion, excepting in the circumference of the original wave which is their common tangent. Or we may suppose a luminous aperture bounded by opaque bodies, where the undulation propagated from the aperture will be always bounded. by the opaque substances; for the secondary waves, without this boundary, will be too feeble to produce light. Hence it follows that, according to this theory, light is propagated in straight lines. (To be continued in our next.)

LIBRARY COLLOQUIES.
No. II.

Editor (reading).

We

EXAMPLE is more powerful than precept, and feeling precedes reflection.
like or dislike without reference to abstract principles. The world did not
want to be told that Shakspere was a great poet, or Raffaelle a great painter-
they were stars that shone by their own light. The critic, indeed, may
question our admiration,—may require a reason for the faith that is in us;
but his place is to follow, not to precede. His comment must wait upon the
text. His province is to examine and decide on results; not to interfere in
the operations by which they are produced. He may be the dispenser of fame,
but he must not assume to be the guide of genius. It is surely just that it
should be so. The poet and the artist produce their testimonials; they bring
forward proofs of their qualification for the office they undertake; the poem
and the picture--the statue and the structure are before us, and submitted to
our judgement: but who is to answer for the Winkelmans and the Webbs,
the Scaligers and the Bentleys of our day? Upon what grounds do they
claim our confidence, and where are the credentials which authorise them to
lay down the law? "Longinus," we are told by Pope, was himself the
great sublime he drew:" but I am not aware that any of his successors can
claim our submission by a similar title. We must recollect also that the same
poet says:-

"Let those teach others, who themselves excel,
And censure freely, who have written well."

"

It may be questioned whether the Iliad would have been much improved, "Had e'en the Stagyrite o'erlooked each line."

We may doubt if Phidias or Apelles would have worked with more effect under critical superintendence-whether the Sistine Chapel would have been benefited by being submitted to the control of a committee of taste-or if Shakspere and Milton would have written much better if their pens had been guided by Bentley, Malone, and Warburton.

No, no, gentlemen! Genius cannot safely take the critic into partnership. That fastidious functionary would soon paralyse the operations of the firm, and chill the ardour of enterprise, by the fear of reproof and the prophecy of failure.

It surely cannot be considered very presumptuous to claim for the poet and the painter the free exercise of their powers, unfettered by the practice or the precedents of other times. I trust it is not quite unreasonable to suppose that Nature may not have so entirely exhausted her gifts on her ancient favourites, but that in her munificence, something has been reserved also for the use of the modern world, which may enable the genius of Great Britain to gain a laurel, even in the Arts, which has not been wholly grafted from the Grecian stock.

Seeing what we have seen in our own day-having witnessed the wonders