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Updated: May 6, 2025
The next chapter deals with an actual physics of colour that is, with the appearance of colours in external space as a result of the refraction, diffraction and polarization of light. The third chapter treats of material colours in relation to chemical and other influences. There, for the first time in the history of modern science, a bridge is built between Physics, Aesthetics and Ethics.
I now invite your attention to the method for testing the flat surfaces on which Prof. Rowland rules the beautiful diffraction gratings now so well known over the scientific world, as also other plane surfaces for heliostats, etc., etc.
These pulsations are not continuous vibrations like the radiations of the spectrum; they are isolated and extremely short; they are, besides, transverse, like the undulations of light, and the theory shows that they must be propagated with the speed of light. They should present neither refraction nor reflection, but, under certain conditions, they may be subject to the phenomena of diffraction.
In addition to this, some of the phenomena exhibited by it, such as diffraction and double refraction, are quite incompatible with the theory that the ether is an elastic solid. The reasons why it cannot be considered as a liquid or gas have been considered previously.
Their slight amplitude, however, is the cause of there here being neither refraction nor diffraction phenomena, save in very special conditions.
The diffraction phenomena obtained during the artificial precipitation of clouds from the vapours of various liquids in an intensely illuminated tube are, as I have elsewhere shewn, exceedingly fine. One of the most interesting cases of diffraction by small particles that ever came before me was that of an artist whose vision was disturbed by vividly coloured circles.
The heat contained in the diffraction spectrum is, with equal dispersions, barely one-tenth of that in the prismatic spectrum. It had, accordingly, never previously been found possible to measure it in detail that is, ray by ray.
Various effects of irradiation and diffraction were, however, observed by Lowell and W. H. Pickering at Flagstaff; and Davidson was favoured at San Francisco with glimpses of the historic aureola, as well as of a central whitish spot, which often accompanies it. That both are somehow of optical production can scarcely be doubted.
Spottiswoode as the fanlike arrangement of the bands the fan opening out at the red end of the spectrum. The diffraction fringes described in Lecture II., instead of being formed on the retina, may be formed on a screen, or upon ground glass, when they can be looked at through a magnifying lens from behind, or they can be observed in the air when the ground glass is removed.
It would, moreover, be extremely important, and lead, no doubt, to most curious applications; it can be conceived, in fact, that such rays might serve to reveal what occurs in those portions of matter whose too minute dimensions escape microscopic examination on account of the phenomena of diffraction.
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