Page:The New International Encyclopædia 1st ed. v. 12.djvu/278

* LIGHT. 252 LIGHT. that there is no opportunity for tlie atoms to vibrate freely. Since the 'cinission spectrum' of a gas is due to the natural frequencies of the atoms, and since the "absorption spectrum' of any body when light of a continuous spectrum falls upon it is due to the absorption of the energj' of those incident waves whose fre<iuencies are the same as those of the atoms, it might lie expected that those waves present in the former are absent in the latter. This is observed to be true, with certain limitations. See K,vdia- Tio.v and Spectuoscopy. Rotation ok the Plane of Polaiuzatiox by Qi.'ABTZ, ETC. A most important connection be- tween matter and the el her is shown liy the fact that wlien plane polarizp<l light is passed through certain bodies — called "optically active' sub- stances — the emerging light is plane polarized, but the plane polarization, i.e. the direction of the vibration, has been rotated. If a Xicol's prism is so placed as to extinguish the plane polarized homogeneous light before the optically active substance is introduced in the path of the light, it will be necessary to turn the Xicol around its axis through a certain angle in order again to extinguish the light after the substance is introduced between the source of light and the Xicol. In some eases the Xicol must be given a right-handed rotation, and in other eases a left-handed one. If the direction of the light is reversed in the l)ody, so is the rotation. Bodies which necessitate the former are called 'dextro-rotatory', and those which require the latter, 'hevo-rotatory.' The amount of rotation of the plane of polarization is different for dif- ferent sulistances: it is different for waves of different color, being greater for shorter waves; it varies directly as the tliickness of the layer of the substance. In some speeiniens of qiarlz the rotation is right-handed; in others, left: and it is found that these two kinds of quartz have a similar dilFerence in their crystalline structure. Turpentine is optically active; so are solutions of certain sugars, two forms of tartaric acid, and all organic compounds involving an 'a.symmetrical carbon atom.' Eiectro-^Iaoxetic Phenomena. The connec- tion between light and electro-magnetic phenom- ena is shown in several ways: the identity in the properties of "light-waves' and the waves pro- duced by electric oscillations; the rotation of the plane of polarization in a magnetic field; the Kerr phenomenon : the Zeeman effect, etc. It is shown in the article on Electricity that the waves produced by electric oscillations are transverse waves in the ether, and that they have the same velocity as 'light-waves.' They can be polarized, can be made to interfere, can be reflected, refracted, etc. The shortest wave- length so far observed, for waves produced by electrical oscillations, is about two millimeters. It can be shown also that the velocity of ether- waves in any transparent medium should vary as the square root of the reciprocal of the product of the electric and magnetic inductivities of that medium. The values of these quantities (K and fi) for the pure ether can be written K,, and ^„; therefore the index of refraction of the trans- parent medium For all such media /t = /t„; and Ko = 1 on the C. G. S. elec- trostatic system; so n= VK, where K is the value of the electric inductivity (or dielectric constant) of the transparent medium. This ii is the inde.x of refraction for cvry tuny waives, because K is always measured by methods which involve slow oscillations. This formula is veri- fied by experiment in many cases. As noted also in the article on Klectbicity, when a beam of plane polarized light is made to pass along the axis of a powerful electro- magnet, the plane of polarization is rotated in the direction in which the current in the mag- netizing helix is llowing. This is called the Faraday effect. So, if the direction of the beam of light is reversed, the rotation is not. The amount of the rotation varies greatly with the transparent matter which occupies the magnetic field; it is proportional to the strength of the magnetic field ; it varies with the wavelengtli and with the temperature. It was stated in speaking of polarized light that if plane polarized light is incident upon a plate of polished metal, the reflected light is elliptieally polarized. Kerr discovered that, if the metal surface is one end of a magnelized bar of iron, the reflected light is not the same as from a surface of unmagnetized iron, lie found further that the changes were opposite in kind if the reflection took place from first a north pole and then a south one. Changes are also ob.served when the light is transmitted through thin films of magnetized iron. Zeeman Effect. A most important discovery, made by Zeeman, has led to a series of interest- ing observations. He has shown that the light emitted by a flame when placed in a magnetic field is different from that emitted ordinarily, and further that there are differences depending upon wlictlier the flame is viewed along the field of magnetic force or at right angles to it. These observations seem to prove that the ether-waves emitted by the flame are actually produced liy the vibrations of minute portions of niattci electrically charged, which have been called 'electrons.' (In this connection reference should be made to the Re/iorts of llir Inlcnuiliomil Von- i/rms of Phi/nicfi, Paris, 1000, vol. iii.) Bibliography. Preston, Throry of Light (Lon- don and Xew York, 1895), is to be recommended as an accurate and interesting treatise on optical theory for the student possessing some knowledge of mathematics. Other works to be recommended are: Drude, Lehrbuch rffr Optih- (Leipzitr 1900. English trans, by Mann and llillikan. Xe« York, 1902) ; Crew. Wave Theory of Lipht (Xew York, 1900). In the chapters on Optics in Miiller-Pouillet. Lehrhiich der Pliysik. vol. ii. (9th ed., Brun.swick, 1897). and Wiillner, Lein- hueh der ETperimentnlphi/sil:. vol. iii. (5th ed., Leipzig, 1900). will be found complete descrip- tions and explanations of optical phenomena. MGHT, Aberration of. See Aberration of Light. LIGHT, EA.SEMENT of.. The right to the f r. ^ access of light to one's windows from and over the land of another. The right to light arises only by grant or by a prescriptive use and enjoy- ment for a long period of time. When acquired in the latter way it is in England kno^^'n as the easement of ancient lifihtfi (q.v. ). In the fnited States it can in general be acquired only by grant, it being regarded as incompatible with the conditions which commonly prevail in this coun- try to permit its acquisition by mere uses and