transmitting those of another. "For if a thinned or plated body which, being of an immense thickness, appear all over of one uniform colour, should be slit into threads, or broken into fragments of the same thickness with the plate or film, every thread or fragment should keep its colour, and consequently a heap of such threads should constitute a mass or powder of the same colour which the plate exhibited before it was broken: and the parts of all natural bodies being like so many fragments of a plate, must, on the same grounds, exhibit the same colour."

Such is the theory of the colours of natural bodies, stated as clearly and briefly as we can. It has been very generally admitted by philosophers, both of our own and of other countries, and has been illustrated and defended by a French philosopher of distinguished eminence. That this theory affords the true explantion of certain colours, or, to speak more correctly, that certain colours in natural bodies are the colours of thin plates, cannot be doubted; but it will not be difficult to show that it is quite inapplicable to that great class of phenomena which may be considered as representing the colours of natural bodies.

The first objection to the theory of Newton is the entire absence of all reflected light from the particles of transparent coloured media, such as coloured gems, coloured glasses, and coloured fluids. This objection was urged long ago by Mr. Delaval, a philosopher of great iepute, who placed coloured fluids on black grounds, and never could perceive the least trace of the reflected tints. Sir David Brewster repeated the experiment with every precaution, and with every variation that he could think of, and he considered it as an established fact, that in such coloured bodies the complementary reflected colour cannot be rendered visible. If the fluid, for example, be red, the green light from which the red has been separated, ought to appear either directly by looking into the coloured

mass, or ought to be recognised by its influence in modifying the light really reflected; but as it cannot be seen, we must conclude that it has not been reflected, but has been destroyed by some other property of the coloured body.

A similar objection may be drawn from the disappearance of the transmitted complementary colours, in the leaves of plants and petals of flowers. It has been ascertained from numerous experiments, that the transmitted colour is invariably the same with the reflected colour, and the same holds true with the coloured juices expressed from them. The complementary tints are never seen, and when there has been anything like an approximation of two tints, it has been invariably found that it arose from there being two coloured juices existing in different sides of the leaf.

In the phenomena of the light transmitted by coloured glasses, there are some peculiarities which we think demonstrate that their colours are not those of thin plates. The light, for example, transmitted through a particular kind of blue glass, has a blue colour of such a peculiar composition, that there is no blue in any of the orders of colours in thin plates which has any resemblance to it. It is entirely destitute of the red rays which form the middle of the red space in the spectrum; so that the particles on which the colour depends must reflect the middle red rays, and transmit those on each side of it-a property which cannot be deduced from the doctrine of Newton.

The explanation of opacity, as arising from a multitude of reflexions, is liable to the same objection which we have urged against the explanation of colour. In order to appreciate its weight, we must distinguish opacity into two kinds, namely, the opacity of whiteness, and the opacity of blackness. Those

bodies which possess the power of reflexion in the highest degree, such as white metals, chalk, and plaster of Paris, never reflect more than one half of the light which falls upon them. The other half of the incident light is, according to Newton, lost by a multitude of reflexions. But how is it lost? Reflexion merely changes the direction of the particles of light, so that they must again emerge from the body, unless they are reflected into fixed returning orbits, which detain them for ever in a state of motion within the body. In the case of black opacity, such as that of coal, which reflects from its first surface only one twenty-fifth of the white light, the difficulty is still greater, and we cannot conceive how any system of interior reflexions could so completely stifle 24-25ths of the whole incident light, without some of it returning to the eye in a visible form.

In determining the constitution of bodies that produces transparency and blackness, the theory of Newton encounters a difficulty which its author has by no means surmounted. Transparency, as we have already seen, arises from the "particles and their interstices being too small to cause reflexions in their common surfaces," that is, they must be "less than any of those which exhibit colours," or "less than is requisite to reflect the white and very faint blue of the first order." But this is the very same constitution which produces blackness by reflexion, and in order to explain the cause of blackness by transmission, or black opacity, Newton is under the necessity of introducing a new principle.

"For the production of black," says he, "the corpuscles must be less than any of those which exhibit colours. For at all greater sizes there is too much light reflected to constitute this colour. But if they be supposed a little less than is requisite to reflect the white and very faint blue of the first order, they will reflect so very little light as to appear intensely black,

and yet may perhaps variously refract it to and fro within themselves so long, until it happens to be stifled and lost, by which means they will appear black in all positions of the eye, without any transparency." In the same paragraph, when speaking of black bodies becoming hot, and burning sooner than others, he says that their "effect may proceed partly from the multitude of refractions in a little room, and partly from the easy commotion of so very small corpuscles."

This very remarkable pasage in his "Optics" exhibits, in a striking manner, the perplexity in which our philosopher was involved by the difficulties of his subject. As the particles which produce blackness by reflexion are necessarily so small as to exclude the existence of any reflective forces, he cannot ascribe the loss of the intromitted light, as he does in the case of white opacity, to a "multitude of reflexions;" and, therefore, he is compelled to have recourse to refracting forces to perform the same office. The reluctance with which he avails himself of this expedient, is well marked in the mode of expression which he adopts; and it is almost certain that when he wrote the above passage, he felt the full force of the objections to this hypothesis, which cannot fail to present themselves to every discriminating reader. As the size of the particles which produce blackness are intermediate between those which produce transparency and those which produce colour, approaching closely to the latter, it is difficult to conceive why they should refract the intromitted light, while the greater and smaller particles, and even those almost of the same size, should be destitute of that property. It is besides not easy to understand how a refraction can take place within bodies which shall stifle all the light, and prevent it from emerging. Nay, we may admit the existence of such refractions and yet understand how, by a com

pensation in their direction, the refracted rays may all emerge from the opaque body.

The force of these objections is tacitly recognized in Pemberton's View of Sir Isaac Newton's Philosophy; and as Newton not only read and approved of that work, but even perused a great part of it. along with its author, we may fairly consider the opinion there stated to be his own.

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"For producing black, the particles ought to be smaller than for exhibiting any of the colours, viz., of a size answering to the thickness of the (soap) bubble, whereby reflecting little or no light it appears colourless; but yet they must not be too small,' for that will make them transparent through deficiency of reflexions' in the inward parts of the body, sufficient to stop the light from going through it; but they must be of a size bordering upon that' disposed to reflect the faint blue of the first order, which affords an evident reason why blacks usually partake a little of that colour." In this passage all idea of refraction has been abandoned, and that precise degree of size is assumed for the particles which leave a small power of reflexion, which is deemed sufficient to prevent the body from becoming transparent; that is, sufficient to render it opaque or black.

The last objection which we shall state to this theory is one to which we attach great weight, and, as it is founded on discoveries and views which have been published since the time of Newton, we venture to believe, that, had he been aware of them, he would never have proposed the theory which is now under consideration.

When light falls upon a thin film so as to produce the colours of thin plates, it follows, from Sir Isaac Newton's theory of fits, that a portion of the light is, as usual, reflected at the first surface, while the light that forms the coloured image is that which is reflected from the second surface, so that all the colours of thin