452 LIGHT FIG. 32. tact; and consequently that a single ray of common light, incident externally in the above mentioned direction, should be divided into an infinite number of refracted rays within the crystal. Here are two singular and unexpected consequences of Fresnel's theory, not only un- supported by any facts hitherto observed, but even opposed to all the analogies derived from experience ; here are two remote conclusions of that theory deduced by the aid of a refined analysis, and in themselves so strange that we are inclined at first to reject the principles of which they are the necessary consequences. They accordingly furnish a test of the truth of that theory of the most trying nature that can be imagined." Dr. Lloyd, at Sir William Hamilton's request, made the fol- lowing experiments : There were two cases in which it was ex- pected cones of light would be produced. The first was that of external conical re- fraction. A plate of aragonite, a biaxial crystal, was prepared with its faces perpendicular to the line o p, fig. 32, bisecting the optic axis, which in ara- gonite contains an angle of about 20. "A thin metallic plate perforated with a very mi- nute aperture was placed on each face of the crystal, with one aperture at o and the other at m. The flame of a lamp was then brought near one of the apertures, and in such a position that the central ray of the converging beam should have an incidence of 15 or 16. When the adjustment was completed a brilliant an- nulus of light (fig. 33) appeared on looking through the aperture in the second surface. When the ap- erture in the second plate was very slightly changed so that the line connecting the two apertures no longer coincided with the line m o, the phenom- ena rapidly changed and the annulus resolved itself into two separate pencils." It was found that the rays composing the emergent cone were all polarized in different planes, which are connected by the following law : " The angle between the planes of polariza- tion of any two rays of the cone is half the angle between the planes containing the rays themselves and the axis." The law was dis- covered by observation, but may be deduced from Fresnel's theory. u The other case, that of internal conical refraction, was expected to take place when a single ray has been incident externally upon a biaxial crystal in such a man- ner that one of the refracted rays may coincide with an optic axis. The incident ray in this case should be divided into a cone of rays within FIG. FIG. 84. the crystal, the angle of which, in the case of aragonite, is 1 55'. The rays comprising this cone will be refracted at the second surface in directions parallel to the incident ray so as to form a small cylinder of rays in air whose base is the section of the cone made by the surface of emergence. This is represented in fig. 34, in which n o is the incident ray, a o & the cone of refracted rays within the crystal, and a a' & &' the emergent cylinder." This experiment was more difficult than the other. Suffice it to say that when the required posi- tion was attained the two rays into which the incident ray was divided " suddenly spread out into a continuous circle." The experiment was repeated with the sun's light, and the cylin- der received on a screen at various distances, but with no change in the size of the section. The ob- served angle of the cone was 5' less than the theo- retical, 1 55'. The rays of the internal cone are all polarized in different planes, and governed by the same laws as in the other case. Polarization of Heat. The rays of heat being identical in nature with those of light, it might be supposed that they would be governed by similar laws of double refraction and polarization, and this has been found to be the case. The first experiments were made by Malus and Berard in 1810. By using a piece of rock salt formed in the shape of a rhombohedron similar to Fresnel's rhomb, of St. Gobain glass, Forbes found that heat, like light, is circularly polarized. It has also been shown by Knoblauch and others that the rays of heat suffer diffraction and interference like those of light. CHEMICAL ACTION OF LIGHT. A great many substances undergo chemical change when exposed to the light of the sun, or to that of certain artificial sources. This is explained upon the undulatory theory by sup- posing that the kinetic energy of the molecules of ether is transferred to the molecules of the substance in such a degree as to cause them, to be shaken asunder. The measurement of the chemical action of light and the investi- gation of its laws were successfully commenced by Dr. John W. Draper of New York about the year 1840. He employed for the pur- pose of measurement a reaction originally observed by Gay-Lussac and Thenard, which takes place in a mixture of chlorine and hy- drogen gradually in diffuse, explosively in di- rect sunlight. His apparatus enabled him to determine the amount of hydrochloric acid which would be produced in a given time with given volumes of the gases; and although these were pioneer experiments, they led him to the first great law of photo-chemical action, viz. : *' that the chemical action of light varies in direct proportion to its intensity, and to the time of the exposure." The subject has been
