This very remarkable passage exhibits, in a striking manner, the perplexity in which our author was involved by the difficulties of his subject. As the particles which produce blackness by reflection 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 reflections;” 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 I am persuaded that when he wrote the above passage, he felt the full force of the objections to this hypothesis, which cannot fail to present themselves. 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 compensation in their direction, the refracted rays may all emerge from the opaque body. The force of these objections is tacitly recognised in Pemberton’s View of Sir Isaac Newton’s Philosophy;[21] 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. “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 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 reflections_ 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 is abandoned, and that precise degree of size is assumed for the particles which leaves a small power of reflection, 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 we are considering.