The explanation of _opacity_, as arising from a multitude of reflections, 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 reflection 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 reflections. But how is it lost? Reflection 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 1/25th of the white light, the difficulty is still greater, and we cannot conceive how any system of interior reflections 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 produce _transparency_ and _blackness_, the Newtonian theory 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 reflections 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 reflection, and in order to explain the cause of blackness by transmission, or black opacity, Newton is obliged to introduce 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[20] 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.”