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ray of solar light is composed of seven coloured rays, each possessing a different degree of refrangibility or susceptibility of being turned from its natural course in a straight line by the interposition of a refracting medium. Thus the red ray, being the least refrangible, is found nearest the normal course of the pencil of light, or lowest on the screen, while the violet ray, being most refrangible, is thrown farthest from the normal course, or highest up on the screen; the intermediate colours, in order of increasing refrangibility, being the orange, yellow, green, blue, and indigo. Since this discovery by Newton, it has been contended that instead of seven colours, there are but three—red, yellow, and blue, each extending with varying intensity throughout the whole length of the spectrum, and each displaying its greatest brilliancy at that point where its colour is least confused with its neighbour’s: these have therefore been denominated the primary colours, the secondary tints being produced by the blending together of less intense portions of the primaries.
Each of these rays has its own distinct and peculiar properties, and each exercises its individual function of the solar influence. The red ray, called the calorific, is that through which we receive heat. Its superiority in this respect over the other rays was proved by the experiments of Herschel, who placed delicate thermometers in all parts of the spectrum, and found that during ten minutes that placed in the red ray rose 5° above that in the violet ray; but, what will seem more curious, the highest temperature was found to exist at a short distance beyond the red ray, and out of the visible range of the spectrum.
The yellow is called the luminous or light-giving ray. It is the most intensely brilliant of the whole spectrum, and hence has the power of exciting the optic nerves to a greater degree than its companions; but, so far as is known at present, it has no invisible property.
The extraordinary influence exerted by the blue ray on all objects of the light-receiving world, and its power of acting upon certain chemical preparations—from which the art of photography took its rise—have earned for it the title of the actinic or chemical ray. It is the most subtle, but most potent of them all, for to its pervading influence we owe most of the benefits and enjoyments we receive, directly or indirectly, from the solar light: the verdure of spring and the sereness of autumn, the odour of the violet, the blush on the rose, the golden glory of the ripening corn, and the copious juices of the luscious fruit, are all developed by its latent power: it is its silent and unseen agency that vivifies the vegetable, and invigorates the animal world; that flushes with the bloom of health the cheek of beauty, and swarths the sweating brow of the toiling husbandman. Light we can produce, and heat we can generate by artificial means, but we cannot counterfeit that “kindest ray” under whose benign aspect
“Reviving sickness lifts her languid head;
Life flows afresh; and young-eyed health exalts
The whole creation round.”
