442 LIGHT FIG. 10. diagonal of the face of the crystal, supposing its edges to be of equal length. The ordinary ray follows the law of sines as in single re- fraction, but the extraordinary ray does not, except when the plane of incidence is perpen- dicular to the axis of the crystal ; in which case, however, the
indices of refrac-
tion differ, the or- dinary index being 1-66, while the ex- traordinary is 1'52. Huygens's explana- tion of double re- fraction is con- tained in the fifth chapter of the Tractatus de Lumine, and one of the remarkable geo- metrical constructions contained in it may be briefly stated as follows : Let A 0, fig. 10, be the incident ray, and OF the surface of the crystal. Produce AC to B, and draw B F perpendicular to it, meeting the surface in F. Let CD : OB :: sine of refraction : the sine of incidence of the ordinary ray ; and from the centre 0, with a radius CD, describe the spherical surface DOG. Describe the sphe- roid of revolution G E with the same centre, its axis of revolution being in the direction of the optic axis of the crystal and equal to the diameter of the sphere, the other axis being greater in the ratio of the ordinary to the ex- traordinary index. If from a line perpendic- ular to the plane of the diagram at F tangent planes F O and F E be drawn to the sphere and spheroid, the lines O and E drawn from the centre to the points of contact will be the directions of the ordinary and .extra- ordinary rays. In Iceland spar and many other crystals the index of refraction of the extraordinary is less than that of the ordinary ray, but there are other crystals which refract the extraordinary ray the most. The class of crystals to which Iceland spar belongs are called negative, while those which refract the extraordinary ray the most are called positive crystals, both classes being uniaxial. . The fol- lowing is a list of double-refracting crystals : negative uniaxial crystals Iceland spar, spath- ose iron, tourmaline, sapphire, ruby, emer- ald, apatite, pyromorphite, ferrocyanide of potassium; positive uniaxial crystals zircon, quartz, apophylite, titanite, boracite, ice; bi- axial crystals nitrate of potash, sulphate of iron, sulphate of barium, Brazilian topaz, sugar, selenite, aragonite, strontianite, kyanite, epidote, mica. Interference. The important principle now known under the name of in- terference of light was first proposed by Dr. Thomas Young more than half a century ago. This class of phenomena result from the mu- tual interference of waves of light when they proceed from two neighboring sources and meet each other under a very small angle, and may be shown by the experiment of Gri- maldi, which is described further on in con- nection with diffraction. The experiment of Grimaldi is not however satisfactory, as inter- ference takes place in consequence of diffrac- tion from the action of the edge of the aper- ture. Young modified the experiment so far as to afford him the means of establishing the law of interference, but it was not freed from the objections which might be urged, that the effects were also produced, as in Grimaldi's experiment, by the edge of the aperture. Fres- nel afterward made the experiment in such a way that interference took place without the possibility of diffraction, and his experiment is regarded as one of the most instructive and elegant in the range of physics, and as a demonstration of the truth of the undulatory theory. He employed two mirrors placed to- gether at a very obtuse angle (a very little less than 180), and reflected from their surfaces upon a screen light from the focus of a lens, in such a manner that on reaching the screen some of the undulations of two converging rays should correspond and intensify one another, while others should be separated by half a wave length and destroy one another. Dif- fraction. A divergence of the rays of light in passing the edge of an opaque body in such a way as to produce interference is called dif- fraction. The phenomena were first observed and partially explained by Grimaldi, an Italian physicist, and published in a work entitled Physico-mathesis de Lumine, Coloribus et Iride aliisque Annexis, in 1665, two years after his death. He noticed that the circle of light formed upon a screen when rays were passed through a minute orifice into a dark chamber was bounded by fringes which ex- tended into the shadow beyond the geometrical projection. Again, admitting light through two small apertures sufficiently near together to cause the pencils of light projected upon a screen to overlap each other, he further ob- served that although the space occupied by the overlapping was more brightly illuminated, its borders were darkened by bands or fringes to a greater degree than the other parts of the spectrum. From these discoveries was de- duced the proposition that light added to light may produce darkness. To observe the phe- Fio. 11. Diffraction. nomena of diffraction with greater advantage than was possible with Grimaldi, who was not aware of the compound nature of light, mo^ nochromatic light should be employed. Let a beam of light be received into a dark room, fig. 11, and place a plate of red glass in the
