In practice this apparent difficulty in overcoming the initial resistance with a strong current is easily overcome by the use of a "boosting coil," which supplies the strong current for an instant, and is then shut off automatically, the ordinary current continuing for producing the light. The mechanism is hardly more complex than that of the ordinary incandescent light, but the current of ordinary strength produces an illumination about eight times as intense as the ordinary incandescent bulb of equal candle-power. 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