If we pass an electric current througlh any coniductor, as a cylindrical wire, its temperature tends to increase by the transformation of electrical energy into heat. If no lheat is allowed to escape, the temperature will increase indefinitely, or until tlle conducetor is mlelted or otherwise destroyed. If the lheat is allowed to escape by radiationi alonie from the suirface of the conductor, its temperature will increase only until the rate of loss by radiation is exactly equal to the iate of

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... ransformation. By lheat we htere include all radiant energy, wlhetlher of higlh or low degree. As the temperature of an inicandescenit body increases, lot only does the actual quantity of radiant energy increase, but its wave lengthis diminish. Hence, as experienice has shown, after incandescence is reached, iniereasing the temperature in a given ratio, increases the liglht emitted in a muchl greater ratio. The exact relation between temperature and luminosity is not know:n, aiid it is probably not very simple, if such a relationi exists at all. The phlenomienon of luminosity is really a physiological one and depends partly upon ilndividual optical capacity; some persons being able to see above, and some below the visible spectrum of the average lhuman eye. The radiant energy we call light is one thing. The sensation of luminosity by wllichl we always estimate light is an entirely different thing. A constant source of light may vary greatly in luminosity, according to the conditioni of the receptive mechanisnm and its individuality. But even ignoring the plhysiological aspect of the question,