Scientific American 9, 29.8.1863
The following speculations on light are by D. 8. Barnard, in the Photographie News (London):-
"'As white as fine linen,' 'as white as snow,' are frequent comparisons; but they are all dull examples as compared to many chemical precipitates. Precipitated chalk far outshines the natural varieties, and fine qualities of magnesia carbonate surpass this. Microscopic examination indicates that this latter consists of particles, clear and colorless, but very minute. White lead consists of particles equally minute and also transparent, but of a yellow brown color by transmitted light; consequently, when seen in bulk it appears of a loss pure white. But magnesia cannot be used as a pigment because it possesses no body; and the difference between the white lead and the magnesia in this respect depends upon the different refractive powers of the individual particles which compose the separate powders. They are both transparent in their individual particles, but the magnesia is more so. They are both bodies possessed of considerable refractive power, but the lead is more so. When air intervenes between their particles the reflective power of both so much exceeds that of air, that they are highly reflecting and very slightly transmitting; but the less absorbing power of the magnesia makes it the whitest - the more reflecting of the two. But when oil intervenes, as would be the case if they were used for pigments, the refractive power of the magnesia so nearly coincides with that of the oil, that much transmission and little reflection is the result, and this constitutes what painters call want of body. But the lead so greatly exceeds the oil in refracting power that its reflective property is not much interfered with, and even with its greater absorbing power it reflects much and transmits little light and this is what painters call great body.
"The length of an undulation of violet light is seventeen millionths of an inch; the red undulation is twenty-six millionths; undulations longer or shorter than these not being visible. Again, the length of the light wave varies in the medium. An undulation in air measuring four will measure only two and a half when it enters glass, and will again elongate to its former measure on its exit. When an undulation passes from air into water, or into the humors of the eye it likewise becomes shortened. If we say that luminous undulations, which in air measure twenty-two millionths of an inch, look yellow when they enter the eye (that being the wave length belonging to what we call yellow light), we must also remember that they measure one-third less in that organ in consequence of its refracting power. We then come to the singular couclusion that the blue sky is yellow, sunshine is red, and the rosy tints of evening are not luminous at all till they enter the eye. If the color depends upon the length of the light wave, and the length of the wave depends upon the refracting power of the medium through which it is passing, every beam of light changes color; red it may be on passing through the region of tho stars, yellow or green it may be when it enters our earth's atmosphere, blue or violet when it enters water, non-luminous as it passes through glass. But if light, which we perceive as violet while it exists in the agneons humor of the eye, was red originally, what color must that light be which we perceive is red? Its undulations in air must be too long to be luminous at all. This introduces us to the solemn thought that all this vast universe is dark! Light only exists in the eye. It is only a sensation - a perception of that which in nature exists as a force capable of producing a sensation."