To thus examine the constituents of a beam of light a lantern, with a lime-light or electric light, may be used for throwing a constant beam; we may then produce an image of the cylinders of lime or the carbon points in the lantern on a piece of paper or a screen, and our eyes will tell us that this is an instance of how the radiations from any incandescent substance are competent to give us light. We receive all the rays to which our eyes are tuned and we see a white image on the screen. We shall see also that the light is more intense than that of a candle, in other words that the radiation from the light-sources we have named is very great. [Illustration: FIG. 164.—Thermopile and Galvanometer. ] Now let us insert in front of the lantern a piece of deep red glass, that is, glass which allows only the red constituents of the white light to pass. Now if a thermo-electric pile, Fig. 164, be introduced into the beam we shall see that the needle of the galvanometer will alter its position. Now, why does the needle turn? This is not the place for giving all the details of this instrument, but it is sufficient to say (1) that the needle moves whenever a current of electricity flows through the coil of wire surrounding the needles, and (2) that the pile consists of a number of bars of antimony and bismuth joined at the alternate ends, and whenever one end of the pile is heated more than the other, a current of electricity is caused to flow. Such is the delicacy of the instrument, that the heat radiated from the hand, held some yards away from it, is sufficient to set the needle swinging violently; this then acts as a most delicate thermometer. In this case it shows that heat effects are produced by the red constituents of the light from the lamp. Now replace the thermopile by a glass plate coated with a salt of silver in the ordinary way adopted by photographers. No effect will be produced. Replace the red glass by a blue one. If the light is now allowed to fall on the photographic plate, its effect is to decompose, or alter the arrangement of, the atoms of silver, so that on applying the developing solution, the silver compound is reduced to its metallic state on the places where the light has acted; and thus, if the image of the light-source has been focussed on the plate, a photograph of it is the result. If the thermopile is brought into the beam it will be now as insensitive to the blue light as the photographic plate was to the red light in the former case. We have therefore three kinds of effects produced, viz., light, heat, and chemical or actinic action, and when light is passed through a prism, these three different radiations, or energies, are most developed in three different portions of the spectrum.