The form of lamp used is that of a long, horizontal tube suspended overhead in the room, a brilliant light being diffused, which, lacking the red rays of ordinary lights, gives a bluish-green tone to objects, and a particularly ghastly and unpleasant appearance to faces and hands, as referred to a moment ago. In many ways this feature of the light is really a peculiarity rather than a defect, and for practical purposes in work requiring continued eye-strain the absence of the red rays is frequently advantageous. In such close work as that of pen-drawing, for example, some artists find it advantageous to use globes filled with water tinted a faint green color, placed between the lamps and their paper, the effect produced being somewhat the same as that of the mercury-vapor light. For such work the absence of the red rays of the Hewitt light would not be considered a defect; and in workshops and offices where Mr. Hewitt's lamps are used the workmen have become enthusiastic over them.
On the other hand, the fact that the color-values of objects are so completely changed makes this light objectionable for ordinary use; so much so, in fact, that the inventor was led to take up the problem of introducing red rays in some manner so as to produce a pure white light. He has partly accomplished this by means of pink cloth colored with rhodium thrown around the glass; but this causes a distinct loss of brilliancy.
The most natural method of introducing the red rays, it would seem, would be to use globes of red glass; but a moment's reflection will show that this would not solve the difficulty. Red glass does not change light waves, but simply suppresses all but the red rays; and since there are no red rays in the mercury-vapor light the result of the red globe would be to suppress all the light. Obviously, therefore, this apparently simple method does not solve the difficulty; but those familiar with Mr. Hewitt's work will not be surprised any day to hear that he has finally overcome all obstacles, and produced a perfectly white light. In the meantime the relatively expensive arc light and the incandescent bulb with its filament of carbon or metal hold unchallenged supremacy in the commercial field.
[XII]
THE MINERAL DEPTHS
Ages before the dawn of civilization, primitive man had learned to extract certain ores and metals from the earth by subterranean mining. Such nations as the Egyptians, for example, understood mining in most of its phases, and worked their mines in practically the same manner as all succeeding nations before the time of the introduction of the steam engine. The early Britons were good miners and the products of their mines were carried to the Orient by the Phœnicians many centuries before the Christian era. The Romans were, of course, great miners, and remains of the Roman mines are still in existence, particularly good examples being found in Spain.
Even the aborigines of North America possessed some knowledge of mining, as attested by the ancient copper mines in the Lake Superior region, although by the time of the discovery of America, and probably many centuries before, the interloping races of Indians who had driven out or exterminated the Lake Superior copper mines had forgotten the art of mining, if indeed they had ever learned it. But the fact that their predecessors had worked the copper mines is shown by the number of stone mining implements found in the ancient excavations about Lake Superior, these implements being found literally by cart loads in some places.
The great progress in mining methods, however, as in the case of most other mechanical arts, began with the introduction of steam as a means of utilizing energy; and another revolution is in rapid progress owing to the perfection of electrical apparatus for furnishing power, heat, and light. Methods of mining a hundred years ago were undoubtedly somewhat in advance of the methods used by the ancients; but the gap was not a wide one, and the progress made by decades after the introduction of steam has been infinitely greater than the progress made by centuries previous to that time.