loss of heat by transmission through the glass, the temperature at the focus should be the same as that of a point placed at such a distance from the sun that the solar disk would seem just as large as the lens itself viewed from its own focus.
The most powerful lens yet constructed thus virtually transports an object at its focus to within about 250,000 miles of the sun's surface, and in this focus the most refractory substances—platinum, fireclay, the diamond itself—are either instantly melted or dissipated in vapor. There can be no doubt that, if the sun were to come as near us as the moon, the solid earth would melt like wax.
In 1878 Professor Langley made a careful comparison between the radiation of the sun and that of the molten metal in a Bessemer "converter" when at its greatest heat. By a very ingenious arrangement he brought the solar heat and that from the metal to confront each other upon the faces of a thermopile; and he found that, even neglecting all corrections for the loss of solar heat by transmission through the smoky atmosphere of Pittsburgh, and by the reflections which brought it to his apparatus, the sun's radiation was eighty-seven times as powerful as that from the converter, surface for surface. Had the just corrections been ascertained and applied (a matter, however, of extreme difficulty, and even impossible under the circumstances), the ratio would be increased from eighty-seven to more than one hundred certainly, and perhaps to one hundred and fifty.
As to the temperature of the metal in the converter. Professor Langley considers that it must have been above that of the fusion of platinum, because platinum wire held over the mouth of the converter just before pouring, or in the stream of metal, melts immediately. There may be some question, however, whether the melting of the wire really indicates quite so high a temperature, since fluid iron and its vapor attack platinum in something the same way as mercury and its vapor attack gold and silver. Similar conclusions as to the intensity of the solar temperature follow from investigations by Soret and others, as to the penetrating power of the sun's rays; and from a comparison with artificial sources of heat in respect to the relative proportion of the rays of different wave-lengths in the total radiation. A body of low temperature emits an enormous proportion of slowly vibrating, invisible vibrations, while, as the temperature rises, the shorter waves become proportionally more and more abundant. Thus, in the composition of a body's radiation, we get some clew to its temperature. Hitherto all such tests concur in putting the sun's temperature high above that of any known terrestrial flame.
And now we come to questions like these: How is such a heat maintained? How long has it lasted already—how long will it continue—are there any signs of either increase or diminution?—questions to which, in the present state of science, only somewhat vague and unsatisfactory replies are possible.