We know, of course, that aqueous vapors are partially opaque to heat rays, as the radiated heat of the earth is partially arrested by such vapors in the atmosphere, but they are apparently transparent to the rays of light. But we know that this cannot be entirely true in fact, for light rays only differ from heat rays in the comparative length of their waves or impulses, while rays of light are always accompanied--when emitted by a thermally incandescent body--by a much larger number of those of heat. As a body is raised in temperature radiant dark rays first appear; these being raised higher, become visible as light, and new dark rays are radiated behind them, and this continues till after the state of highest incandescence is reached and the invisible chemical rays beyond the spectrum appear. It is like a crowd surging forth in flight from the doors of a building; as the speed of those in front increases to a run, others follow more slowly in the mass, and as these gain speed others continue to follow, while the great mass of laggards still trails along in a lengthening line to the rear. The perception of light is itself merely due to the constitution of the optic apparatus of the observer, which only takes cognizance of vibrations radiated from the middle portion of the scale, just as the ear does with sounds, and not to any actual difference in their mode of production. That heat rays and light rays are identical in constitution can be readily shown by the experiment described by Professor Tyndall in his "Forms of Water," in which an opaque screen of iodine solution in bisulphide of carbon was employed to arrest, in a beam of light, all the light waves (to which it is entirely opaque), while transmitting the dark rays. These non-luminous rays are then converged by a lens: "Let us, then, by means of our opaque solution, isolate our dark waves and converge them on the cotton. It explodes as before.... At the same dark focus sheets of platinum are raised to vivid redness; ... a diamond is caused to glow like a star, being afterwards gradually dissipated." Sir William Herschel (see article "Spectrum," Appleton's Cyclopædia) says, "If we call light those rays which illuminate objects, and radiant heat those which heat bodies, it may be inquired whether light be essentially different from radiant heat. In answer to which I would suggest that we are not allowed by the rules of philosophizing to admit of two different causes to explain certain effects, if they may be accounted for by one."... "Tyndall, by similar experiments, found that the thermal energy of the invisible radiation of a very powerful electric light is eight times that of the visible.... Seebeck showed that the position of maximum heat in the spectrum changes with the nature of the prism and sometimes occurs in the red." Melconi, with prisms of alcohol and water, found it in the yellow. Athermic bands are also found in the heat-spectrum,