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M. Lallemand, however, in a recent communication relative to what he calls the illumination of opaque bodies with a dead surface * (that is to say, on diffusion), seems to me to have taken an important step towards my view. In other interesting ob. servations, he has arrived, for a smoked surface, at a result identical with that which I made known in my previous Note, viz. that the light diffused by lampblack is subject, with respect to its polarization, to precisely the same laws as the light enitted by the trace of a pencil of rays traversing a transparent body t.

Now the lampblack covering a slip of glass, for example, is only an agglomeration of very minute particles in juxtaposition. It seems evident that these particles must continue to diffuse the light according to the same laws, but with less intensity, when, instead of being sufficiently abundant to be contiguous and heaped up on one another, they are more scattered and form only a light deposit on the glass, which partially retains its transparency. This is, in fact, confirmed by experiment. No more can this property be refused to the same particles in suspension in a gas (that is, in the state of smoke or flame), or in suspension in a liquid (for instance, water containing a little Indian ink). It must hence be concluded, therefore, that, given a medium destitute by itself of all power of illumination, it will be sufficient to spread in it very thin particles in order to see produced the phenomenon of the lateral propagation of light polarized according to the laws just mentioned. This is an important point which I sought to demonstrate in my previous researches , and of which I shall presently give fresh proofs.

But if, as I think, we are agreed upon this material fact, we again diverge as regards its interpretation, and even upon the cause of the phenomenon. M. Lallemand considers that, in the smoked surface, it is each molecule of carbon, or rather the atmosphere of' æther condensed around each molecule, that determines the propagation of the light in all directions. For myself, I do

Comptes Rendus de l'Académie des Sciences, May 4, 1874, p. 1272. † It is necessary to remark only that with a smoked surface the polarization is not complete-a fact, besides, often met with in the illumination of transparent bodies. I insisted on this point in my previous Note, and shall return to it further on.

I must here mention that MM. de la Prevostaye and Desains in their memoir“ On the Diffusion of Heat” (Annales de Chimie, 1852, vol. xxxiv. p. 215 et seq.), published some results accordant with these laws, but without enunciating them in a complete manner. Sir D. Brewster, in his memoir entitled “On the Polarization of Light by Rough and White Surfaces (Philosophical Magazine, 1863, vol. xxv. p. 344), did not the case of black bodies.

| See Archives des Sciences, 1870, vol. xxxvii. pp. 150 et seqq.

not go so far, but continue to attribute it to the general fact of the reflection produced at the surface of separation between two unequally refracting media. Each particle of carbon, although very minute, is composed of a great number of molecules; it forms a minute body reflecting light, only, as its dimensions are very sinall, there is no annihilation by interference of the rays emitted in directions different from those determined by the ordinary laws of reflection : there is not specular reflection, but diffusion in all directions.

Let us look at some consequences of these two interpretations. Suppose a liquid having no power of illumination; then introduce some very fine particles of a solid body also having no power of illumination. If M. Lallemand's theory is accurate, the liquid, on ceasing to be homogeneous, will not receive the property of being illuminated, since neither of its constituents possesses it; while, according to my view, the trace of a luminous pencil must be marked in the liquid aud present the usual phenomena of polarization, except in the quite exceptional case of the solid and liquid having the same index of refraction. The experiment is hardly realizable absolutely; but an approximation is possible. Water purified to the highest degree attainable, and containing only very few particles in suspension, has very little illumination-power; and calcspar has no sensible capacity of illumination, the trace of a pencil manifesting itself in the interior only by the slight red fluorescence of this body. Now, on suspending finely pulverized spar in water, and tben filtering the liquor to separate the particles of too great a volume, I have found the power of illumination become much superior to that of the water alone*.

On the other hand, if it is the molecules of the body then. selves that produce the diffusion, it seems to me that the intensity of its manifestation should not depend on the refrangibility of the medium in which we operate. A smoked surface, for instance, whether it be in the air or in a more or less refracting liquid, shou

emit sensibly the same quantity of light. Now this is not what takes place; for it is easy to prove that the diffusing-power of lampblack is less intense in a liquid than in the

* I cannot, however, give this experiment as entirely conclusive. The trace becomes incontestably more evident, and the polarization more pronounced ; but the polarization is not complete, and there remains a residue of neutral light analogous to that which would be produced by an action of fluorescence. The colour of this residue is greenish, while the spar has a red fluorescence; besides, the proportion of solid particles is so trifling that it would be difficult to attribute a sensible fluorescence to them. Further, the trace presents very pronounced differences of colour, according to the angle under which it is viewed- a phenomenon manifested in other cases, and to which I purpose, one day, to return.

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I have meared I my previous Nort, the reasons which. prevent me from believing that this residue is a to fluorescenes. I think that may be explained in part because the lampack is no composed of pure carbon, partly because this

* The conditions are tier ainrost the same as wher lampback deposized of the hinder surface of a Diane of glass is viewed through the glass. winen is traversed by the meltem pencia case of which we have to speak further.

+ Nevertheless this case may present itself: M. Ed. Berquere has in fact observed a phosphorescence in the lampack deposited by zostan fames; but a surface smoked at the flame of benzine is not pirospirovescent.

not go so far, but continue to attribute it to the general fact of the reflection produced at the surface of separation between two unequally refracting media. Each particle of carbon, although very minute, is composed of a great number of molecules; it forms a minute body reflecting light, only, as its dimensions are very small, there is no annihilation by interference of the rays emitted in directions different from those determined by the ordinary laws of reflection: there is not specular reflection, but diffusion in all directions.

Let us look at some consequences of these two interpretations. Suppose a liquid having no power of illumination; then introduce some very fine particles of a solid body also having no power of illumination. If M. Lallemand's theory is accurate, the liquid, on ceasing to be homogeneous, will not receive the property of being illuminated, since neither of its constituents possesses it; while, according to my view, the trace of a luminous pencil must be marked in the liquid and present the usual phenomena of polarization, except in the quite exceptional case of the solid and liquid having the same index of refraction. The experiment is hardly realizable absolutely; but an approximation is possible. Water purified to the highest degree attainable, and containing only very few particles in suspension, has very little illumination-power; and calcspar has no sensible capacity of illumination, the trace of a pencil manifesting itself in the interior only by the slight red fluorescence of this body. Now, on suspending finely pulverized spar in water, and then filtering the liquor to separate the particles of too great a volume, I have found the power of illumination become much superior to that of the water alone*.

On the other hand, if it is the molecules of the body themselves that produce the diffusion, it seems to me that the intensity of its manifestation should not depend on the refrangibility of the medium in which we operate. A smoked surface, for instance, whether it be in the air or in a more or less refracting liquid, should emit sensibly the same quantity of light. Now this is not what takes place; for it is easy to prove that the diffusing-power of lampblack is less intense in a liquid than in the

* I cannot, however, give this experiment as entirely conclusive. The trace becomes incontestably more evident, and the polarization more pronounced; but the polarization is not complete, and there remains a residue of neutral light analogous to that which would be produced by an action of fluorescence. The colour of this residue is greenish, while the spar has a red fluorescence; besides, the proportion of solid particles is so trifling that it would be difficult to attribute a sensible fluorescence to them. Further, the trace presents very pronounced differences of colour, according to the angle under which it is viewed-a phenomenon manifested in other cases, and to which I purpose, one day, to return.

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air. To perceive this it suffices to pour on the smoked surface a drop of alcohol, benzine, sulphide of carbon-in a word, of a liquid which moistens the lampblack; but in this case the polarization-phenomena are impaired*. It is better to operate by immersing the smoked surface in a glass trough containing the liquid. This causes, it is true, a little perturbation owing to the circumstance that the liquids (such as alcohol and benzine) dissolve a substance deposited at the same time as the carbon, and become somewhat fluorescent; but this does not prevent the diminution of the intensity of the diffused light (without alteration of the polarization-phenomena) from being distinctly observed. Is it not probable that, if the index of refraction of the liquid were identically the same as that of the lampblack, there would be no light diffused?

It seems to me useless to insist further on this difference of interpretation, to which I attach only secondary importance. The question of knowing if the molecules can be regarded as by themselves effective centres of vibrations, or if the action of these molecules is transferred to the æther which surrounds them so as to modify its general density in the interior of a particle, is perhaps subtile and premature. To arrive at its solution, a very interesting part of the labours of M. Lallemand must certainly be taken into consideration; I mean his photometric researches. He has, in fact, shown that the intensity of the diffused light can be calculated by admitting that the vibration of the diffused ray is the projection of the incident vibration, and supposing that the vibratory motion is propagated with the same energy in all directions.

On the proper Colours of Bodies.

We have seen that, at the time of the diffusion by a smoked surface, beside the polarized light there is always a residue of neutral light. To recognize it, we have only to view with an analyzer, under a visual angle of 90°, the surface illuminated by a pencil of light, or to examine it under any angle when the incident pencil is previously polarized.

I have indicated, in my previous Note, the reasons which prevent me from believing that this residue is due to fluorescencet. I think that it may be explained in part because the lampblack is not composed of pure carbon, partly because this

* The conditions are then almost the same as when lampblack deposited on the hinder surface of a plate of glass is viewed through the glass, which is traversed by the incident pencil-a case of which we have to speak further.

+ Nevertheless this case may present itself: M. Ed. Becquerel has in fact observed a phosphorescence in the lampblack deposited by certain flames; but a surface smoked at the flame of benzine is not phosphorescent.

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