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Page:Popular Science Monthly Volume 24.djvu/688

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THE POPULAR SCIENCE MONTHLY.

tints, while the secondary, though less vivid, shows its colors in surprising strength and purity.

But the primary bow is accompanied by appearances calculated to attract and rivet attention almost more than the bow itself. I have already mentioned the existence of effective rays over and above those which go to form the geometric law. They fall within the primary, and, to use the words of Thomas Young, "would exhibit a continued diffusion of fainter light, but for the general law of interference which divides the light into concentric rings." One could almost wish for the opportunity of showing Young how literally his words are fulfilled, and how beautifully his theory is illustrated, by these artificial circular rainbows. For here the space within the primaries is swept by concentric supernumerary bands, colored like the rainbow, and growing gradually narrower as they retreat from the primary. These spurious bows as they are sometimes called,[1] which constitute one of the most splendid illustrations of the principle of interference, are separated from each other by zones of darkness, where the light waves, on being added together, destroy each other. I have counted as many as eight of these beautiful bands, concentric with the true primary. The supernumeraries are formed next to the most refrangible color of the bow, and therefore occur within the primary circle. But, in the secondary bow, the violet, or most refrangible color, is on the outside; and, following the violet of the secondary, I have sometimes counted as many as five spurious bows. Some notion may be formed of the intensity of the primary, when the secondary is able to produce effects of this description.

An extremely handy spray-producer is that employed to moisten the air in the Houses of Parliament. A fillet of water, issuing under strong pressure from a small orifice, impinges on a little disk, placed at a distance of about one twentieth of an inch from the orifice. On striking the disk, the water spreads laterally, and breaks up into exceedingly fine spray. Here, also, I have used the spray-producer both singly and in groups, the latter arrangement being resorted to when showers of special density were required. In regard to primaries, secondaries, and supernumeraries, extremely brilliant effects have been obtained with this form of spray-producer. The quantity of water called upon being much less than that required by the rose, the fillet-and-disk instrument produces less flooding of the locality where the experiments are made. In this latter respect, the steam-spray is particularly handy. A puff of two seconds' duration suffices to bring out the bows, the subsequent shower being so light as to render the use of water-proof clothing unnecessary. In other cases, the inconvenience of flooding may be avoided to a great extent by turning on the spray for a short time only, and then cutting off the supply of water. The vision of the bow being, however, proportionate to the

  1. A term, I confess, not to my liking.