SPECTRUM 245 fall. Thus, what in its essential nature is a mere vibratory motion, we may interpret as light if these vibrations fall on the retina, or as heat if they fall on our skin, or as chemical action if they fall upon a photographer's plate. This preliminary conception established, we FIG. T. m readily interpret the various actions of spectrum on different bodies, if we also e into consideration the manner in which spectrum is formed, that is, whether by diffractive action of a grating, or by the lispersive action of a prism. We should also ~ into account the nature' of the body forming the grating or the prism. The upper portion of fig. 7 shows a prismatic spectrum crossed by the principal Fraunhofer lines, from A to the line O. A photometric examination this spectrum shows that the distribution of light in it is represented by the curve B, whose heights above points on the base line are in the ratio of the intensities of the light at corresponding points in the spectrum. The maximum of light is found to exist in the yellow at a point distant from the upper D line one third of the distance of this line from the line E. A thermometric examination of this spectrum will give a distribution of heat throughout its length represented by the curve and the maximum of heat is shown at a )int beyond the extreme red, at a distance jual to that of the line A from the line C. he curve gives the distribution of chemi- action in this spectrum, as found when it falls upon a surface of bromide of silver, maximum of action is about midway be- tween G- and H. It is important to remark lat the curves here given only apply to a spectrum which has been formed by this par- ticular kind of glass and has been received on the surfaces indicated. Thus a prism of different glass would change the place of the maximum heat, and the substitution of anoth- er chemical surface on which the spectrum falls^ will cause a shifting of the place of maximum chemical action and a contraction or expansion of the area chemically affected. Heat Actions of the Spectrum. Sir William Herschel in 1800 first discovered that the heat in the spectrum increased from the violet to the red, and reached its maximum intensitj 7 at a distance beyond the red " in the invisible rays of the spectrum." He projected the re- sults of his experiments in a curve which bears a close resemblance to the one given in fig. 7. He experimented on these "invisible rays," which he was the first to discover, and showed that they were reflect- ed and refracted ac- cording to the same laws that rule in the reflection and refrac- tion of light. He says that " if we may infer the quantity of the efficient from the effect produced, the invisible rays of the sun probably far ex- ceed the visible ones in number." He then condensed these rays by means of concave reflectors and lenses, and made a "trial to render the invisible rays of the sun visible by condensation." He showed "that by con- densation their heating power is proportion- ally increased ; for, under the circumstances of the experiment, we find that it extended so far as to be able to raise the thermometer in two minutes no less than 24." In the same series of researches ("Philosophical Transactions," 1800) he 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 cer- tain effects, if they may be accounted for by one. ... It remains for us only to admit that such of the rays of the sun as have the refrangibility of those which are contained in the prismatic spectrum, by the construction of the organs of sight, are admitted, under the appearance of light and colors ; and the rest, being stopped in the coats and humors of the eye, act on them, as they are known to do on all the other parts of our body, by occasioning a sensation of heat." In 1865 Tyndall repeated the researches which had pre- viously been clearly marked out by Herschel. In his first series of experiments he employed the electric lamp as the source of light and heat, and used a linear thermo-battery (see HEAT) as a thermometer. He used prisms of rock salt instead of glass, and in certain of his experiments he passed the beam from the elec- tric lamp through a tank containing a solution of iodine in carbon disulphide. This solution has the property of absorbing all of the radia- tions producing light, and allowing the invisible rays of heat alone to traverse it. Herschel had already found that the "invisible rays of the sun far exceed the visible in number." Tyndall by similar experiments found that "the thermal energy of the invisible radiation of a very pow-
