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the particles bearing a certain constant direction with respect to that radius; and this is polarisation."
The conception of transverse vibrations, now that the idea has become familiarised, seems to present little difficulty; yet it was at first opposed to the prepossessions even of the most zealous undulationists. Fresnel long hesitated fully to adopt the idea, although admitting it to be the only mode of representing polarisation, on the ground of being unable to reconcile it with mechanical principles; and this more precisely as to the notion of transverse vibrations alone being produced, which constituted this theory in all its simplicity; whereas Young had (as we have just seen) believed both these and longitudinal vibrations to co-exist. To establish this point, he expressly says, was the main difficulty which embarrassed him.*
This idea of vibrations performed in directions at right angles to the line of the ray received at length its decisive proof from the phenomena of the coloured tints developed in polarised light by the interposition of plates of crystals (such as those of mica, selenite, &c.), when examined by an analyser.
Young ascribed these colours generally to interference; but both Fresnel and Arago pointed out that this explanation was incomplete. Why did it only take place in polarised light, and even then not until the analyser had been applied?
These questions could not be answered until another important law had been established by the joint researches of Fresnel and Arago; and this consisted in the experimental conclusion, that when two rays are, in other respects, in a condition to interfere, but are polarised in planes at right angles to each other, they cannot interfere: they can only do so when polarised in parallel planes. Now this result, of necessity, implies that in rays polarised in different planes the vibrations must be executed in different planes; and this involves the admission that the vibrations must be transverse to the ray.
This principle was at length seen to complete the explanation so long sought of the polarised tints. The light originally polarised in one plane was, in traversing the doubly refracting crystal, divided into two portions in planes at right angles, which, as Young had shown in regard to position, were in a condition to interfere; (the ordinary ray of one such pair coinciding in direction with the extraordinary of some other pair :) were it not that being polarised in these rectangular planes, they could not interfere. It only required, then, the action of the analyser to resolve each portion into two, suppressing those in one plane, and transmitting those in the other, which, having their vibrations now parallel,
* Ann. de Chimie, 1831, tom. xvii. p. 184.
were thus enabled to manifest their interference, and produce the polarised tints, which in homogeneous light are simply alternations of light and darkness.
Such was the beautiful train of reasoning followed out by Arago and Fresnel to explain those polarised colours, originally and independently discovered by Arago and by Sir D. Brewster, and which, when the crystal is cut perpendicular to its axis, are seen in the form of rings round that axis; and of which the varieties in different crystals are now become familiar by means of the little instrument called the polariscope. But the principle of transverse vibrations thus established was the fruitful source of numerous other conclusions.
We have dwelt on the principle involved chiefly as bearing on the difficulties already spoken of, as experienced by Young and Fresnel in the explanation of phenomena with which these last would at first sight not appear to have any connection, but which were now shown to have a direct relation to them. According to the principle of transverse vibrations, as investigated by Fresnel, a number of comprehensive formulas resulted, including the deduction of Brewster's law of the polarising angle, as well as other important consequences.
One of these is the simple consideration, that in reflection, an oblique vibration is resolved into two at right angles, and thus a change of plane, or of angular phase, takes place. This is equivalent to a retardation or acceleration in route by a corresponding portion of a wave length; and if in any case the plane were turned through 180°, the ray would be in an opposite phase, or the change would be equivalent to a difference of one half an undulation. Obvious as now seems the conception, that a change of plane or angular phase is equivalent to retardation or acceleration in length of route, it does not appear to have been distinctly apprehended in the first instance either by Young or Fresnel, or it would have obviated some difficulties which long perplexed them; especially in the remarkable instance before mentioned of the colours of thin plates, the central black spot, and the supposed arbitrary assumption of the loss or gain of the half-undulation necessary to explain it.
Although Young, in the very same paper containing his theory of interference, pointed to a mechanical analogy which involved nearly the principle of its explanation, he did not perceive the application of it. Indeed, it can hardly be said to have been very clearly or happily expounded by any of the elementary writers who have systematised the theory, not excepting the author of the biography before us. It is thus less surprising that it should have long continued to furnish a ground of difficulty and objection. Yet in Fresnel's formula a mere change of algebraical sign, in the
sine of an angle, in the expression for the reflections at the first and at the second surface, gives a difference of 180° in phase, which is thus accounted for without any subsidiary assumption whatever.
We have thus far endeavoured briefly to sketch the leading points in the history of the first establishment of those grand principles of the theory of light, which Young has the undivided and unquestioned honour of having been the first to propose and to demonstrate. Every subsequent improvement and enlargement of the theory, which has regularly kept pace with the advance of experimental discovery, has, as it were, grown out of the simple principles at first laid down by a natural sequence, without any new hypotheses or forced and arbitrary changes. It is a theory of which an eminent philosopher, by no means unduly biased in its favour, and at a time when it had not reached its present point of perfection, emphatically said, "It is a series of felicities; and if not true, eminently deserves to be true." And the increasing proof which it continues to receive by its readiness in meeting nearly every new experimental case as it arises, augments in the same proportion our conviction that it will, sooner or later, be equally successful in the solution of those few phenomena which still appear to stand out as exceptional instances to its application.
The First Cause; or, a Treatise upon the Being and Attributes of God. By Rev. J. C. Whish, M.A. Seeley, Jackson, and Halliday. 1855.
The Burnett Prize-Essays.
Principles of Psychology. By Herbert Spencer. Longmans. 1855. George Jacob Holyoake and Modern Atheism. An Essay. By S. D. Collet. Trübner. 1855.
The Reasoner for 1855. Edited by G. J. Holyoake. Holyoake,
Ir ever the dark shadow of Atheism were suddenly to envelop the earth, would the crash of falling churches, the disbanding of ecclesiastical classes, and the vanishing of all conscious individual intercourse with God, be necessarily accompanied by the yielding of all moral ties and the dissolution of every sacred social organisation? Before we can attempt to answer such a question, we must call to mind a very obvious but a strangely-forgotten truth, that
human trust does not create God, and that human distrust would not annihilate Him. There is a thoroughly atheistic way of shuddering over Atheism, which is apt to express itself as if the spread of human disbelief would not only overcloud but empty Heaven. Although the darkness which we have supposed would hide God from us, it would not hide us from God; nor should we ever be beyond the reach of His moral influence. When people assume that an atheist must "live without God in the world," they assume what is fatal to their own Theism. We deeply believe that by far the greater part of all human trust does not arise, as is commonly supposed, from our seeking God, but from God's seeking us; and this, too, without any clear admission or confession on our part of His influence upon us ;—that a great deal of it is trust in goodness rather than in any personal God, and might possibly be held along with intellectual disbelief of His personal existence; in short, that if you could blot out on the one hand all acts of self-confessed trust in God,—if you could blot out all private and public worship, properly so called, spurious or genuine, all churches, all creeds, all pharisaism, and all pure conscious devotion; and if, on the other hand, you might leave all this, and blot out of the earth all unconscious and unconfessed acts of surrender to the divine influence in the heart,—all that might possibly be connected with purely intellectual Atheism,-you would blot out more of true "religion," more of that which "binds together" human society, more of God's true agency on the earth, in the latter case than in the former. Of course we do not mean that the truest unconscious trust in God's influence is not generally to be found in the same minds which, at other times, also consciously confess Him; but only this, that if in every life, whether of faith or doubt, you numbered up the acts of trust which are not rendered to God personally, but to the instincts and impulses which so often represent Him in the heart, and which might continue to represent Him even when the dark cloud of conscious doubt of His existence had intervened, you would probably have numbered far more acts which really originate in divine influence than could possibly be found animated by a real conscious personal belief.
And if this be so, as we believe most men will admit as much from self-knowledge as from knowledge of the world, it is a fatal blunder to attempt to prove to the atheist that, in consequence of his doubt, he has been and is living totally without God; that his eyes need opening, not in order that they may recognise One who has been ever with him, but that they may help him to find a distant and alienated power. There is no teaching more mischievous in its effects than that which makes human belief in God
the first regenerating power in human society, and God Himself the second; which makes God's blessing a consequence of man's confession, and which therefore limits that blessing to the narrow bounds of the confession. In fact, this delusion tends to depress rather than to exaggerate ordinary men's real estimate of the value of faith. Hearing it constantly implied that God influences men's hearts only so far as they confess His influence, that He will do nothing for them, morally and spiritually, unless they render the "glory" where it is due; and yet, seeing that in fact this sine quâ non of divine influence is any thing but a true mark of actual goodness, being often only the crowning element in evil, a school of thought has sprung up which depreciates the value of faith altogether, which delights in discovering that the greatest good is, after all, to be found hidden under a mask of scepticism and self-mockery, in short, a school which replaces the religious ascription of all goodness to God's grace by light ridicule of a human nature that does not pretend to be so assisted, but rather does the best it can for itself in an unostentatious way. This disposition to compare keen self-mockery with formal belief, and to give the preference to the former, is perceptible enough in the whole tone of our literature. Thackeray's writings are throughout tinged with the feeling that thorough self-distrust is one of the highest moral virtues of which men in general are capable. And until even this honest self-exposure, and every other sort of goodness, so far as it is goodness, be shown to be attributable to God's Spirit working in man, far though it be removed from the theological virtue of faith, faith itself will never recover from the discredit into which its undue isolation has brought it. As soon as God is confessed to be far greater than our faith, we shall begin to make the effort to render our faith more worthy of God: but while men own so many things to be noble which are never claimed as divine because they are unaccompanied by this conscious faith, so long they will care little what that faith does or does not include. Men have found the faith-classification of human actions so narrow and unjust, they have seen so much goodness without faith, and so much faith without goodness, that they begin to preach justification by sincerity as a more human, if it is not a more divine formula than justification by faith.
In showing, then, that Atheism is false to human nature, that trust in God is the natural atmosphere of our moral life, we must not take for granted, as is so often done, that belief in God as God, and belief in goodness, are one and the same thing. We must grant the atheist his unexplained impulses to good, the implicit God of his conscience, and show how he mutilates and dwarfs human nature by denying it all explained impulses