Page:The American Cyclopædia (1879) Volume X.djvu/447

LIGHT 441 vapor of a substance has the power of absorb- ing rays which that substance in a state of incandescence emits; so that, there being in the solar atmosphere vapors of various incan- descent bodies, some of the rays originating in the incandescent mass are absorbed, leaving dark lines in their places. These dark lines, having a definite position with regard to the ref rangibility of rays, are employed as a means of marking the wave lengths of the different rays. The following table gives the lengths of undulations in parts of an inch, and also the number of undulations performed in a second, corresponding to the different dark lines and other places in the spectrum, as computed by Fraunhof er : PLACE IN SPECTRUM. Length of undu- lations. No. of undulationt per second. Line B 00002708 451,000,000,000 000 Line C 00002588 473,000,000,000,000 Middle red 00002441 500,000,000,000,000 Line D. 00002319 527,000,000,000,000 00002295 532,000 000 000 000 Middle yellow 00002172 562,000,000,000,000 Line E 00002072 590,000,000,000,000 Middle green 00002016 606,000,000,000,000 Line F 00001906 641,000,000,000,000 Middle blue 00001870 653,000,000,000,000 Middle indigo 00001768 691,000,000,000,000 Line G 00001689 723,000,000,000.000 Middle violet 00001665 733,000,000,000,000 LineH 00001547 789,000,000,000,000 Several of these dark lines were first observed by Wollaston in 1802 ; but as they have since been more completely studied by Fraunhof er, they are called Fraunhof er's lines. He counted over 600 of them, and assigned fixed positions to 354. He selected seven of these as stand- ards of comparison, designating them by the letters B, 0, D, E, F, G, H, of which some are single, some are double, and others composed of a group of fine lines, as at E. Sir David Brewster counted about 2,000, and since then Kirchhoff, Bunsen, and others have extended the number to more than 3,000. In other kinds of light, as that of the fixed stars, flames, and the electric spark, analysis discovers similar bands, but differing in position and magnitude, so that each species of light has its own system of bands, which are distinct physical character- istics. The subject will be treated in the arti- cle SPECTRUM ANALYSIS. Double Refraction. In what has thus far been said about refrac- tion it has been supposed to take place in one direction only for the same medium and the same angle of incidence ; but this is not true for the majority of refracting media, but only of those having a homogeneous or a crystal- line structure alike in all directions. Liquids, annealed glass, and crystals whose fundamental form is the cube, possess only the property of single refraction. All transparent substances of regular form, in which there is an unequal state of compression or cohesion of molecules, possess the property of refracting a beam of light in two directions. Such are crystals of the dimetric and hexagonal systems, and glass which is subjected to unequal pressure in differ- ent directions. There is one direction, called the optic axis, in which a beam of light is not divided by these crystals, and this is also the crystallographic axis. They are therefore called uniaxial crystals. Some crystals have two optical axes, or axes through which there is no double refraction. Such belong to the trimetric, monoclinic, and triclinic systems of crystallization, and are called biaxial crystals. The phenomenon of double refraction was first discovered by Erasmus Bartholinus, a Danish philosopher, in Iceland spar, and his account was published in 1669. A few years after- ward the subject was investigated by Huygens, who succeeded in establishing the general laws under which double refraction takes place. Iceland spar, which possesses the property of double refraction in the most remarkable de- gree, is a variety of carbonate of lime, which substance crystallizes in a great variety of forms, all of which may be reduced by cleav- age to the rhombohedron. If a transparent crystal of the spar be laid upon a printed page, all the letters seen through it will appear dou- ble, but of less depth of color, except where the images overlap. (See fig. 8.) If a line be FIG. 8. Double Kefraction. drawn from one of the solid angles in which three of the obtuse plane angles meet, this line, or any line parallel to it, will be the optic axis of the crystal, which is a direction, and not a particular line. See fig. 9, where a 5, or any line parallel to it, is the optical axis. A ray of ligfyt entering the crystal in the direction of any of these lines will not be divided by refraction, but in any other direction it will be split into two rays separated by an angle of 6 12', one of which was called by Huygens the ordinary, and the other the extraor- dinary ray ; and these possess re- markable proper- ties, as we shall see further on. If a crystal of the spar is laid upon its side over a dot, and ro- tated on an axis perpendicular to the surface on which it lies, one of the images of the dot (the ordinary image) will remain stationary, while the other (the extraordinary) will revolve around it. A line drawn between the two im- ages is always in the direction of the shorter FIG. 9.