POLAlilZATION OF LIGHT. 127 



Stokes, have discovered many others still beyond the violet. By his curious 

 discovery o? fluorescence, or the property possessed by some substances of ren- 

 dering sensible to vision rays beyond the limit of the ordinary spectrum, Pro- 

 fessor Stokes has in fact quadrupled its length.* 



In observing the spectra formed by light from other sources than the sun, as 

 from the fixed stars, from incandescent solids, flames, &c., great differences are 

 found to exist in regard to the lines observed. In the spectra of the iixed stars, 

 dark lines are seen, which, like those of the solar spectrum, are unchangeable 

 ill position, but which do not occupy the same positions. In the spectra of 

 flames, lines are observed Avhich arc not dark, but bright. Different salts added 

 to the wick of an alcohol lamp produce different systems of lines, always bright. 

 So, likewise, metals burned under the compound blow-pipe. The spectrum of 

 the electric spark exhibits bright lines also, the positions and numbers of which 

 depend on the substances of which the electrodes are formed. A platinum wire 

 made incandescent by an electric current, gives no lines at all ; and none are 

 seen in the spectrum formed from the light of the solid carbon electrodes which 

 produce the galvanic arch. Experiments made by Sir David Brewster, by pass- 

 ing solar or artificial light through different colored gases, led him to the con- 

 clusion that the dark lines are caused by absorption — an absorption which be 

 supposed to take place in the earth's atmosphere.! 



In the year ISOS the French Academy of Sciences proposed the problem of 

 the double refraction of light as the subject of a prize to be awarded two years 

 thereafter. The successful competitor for this prize was Mains. To him is due 

 the discovery of the polarization of light by reflection. He was led to this re- 

 markable discovery by an accident. In observing through a prism of Iceland 

 spar the light reflected to his windows from those of the palace of the Luxem- 

 bourg, he was surprised to see that, as he turned the prism around the ray, one 

 of the two images vanished at every quarter revolution. By following up th3 

 indication thus given, he arrived at the important law that, when light is re- 

 flected from glass at an angle of 54° 35', or from water at an angh; of 52^ 45', 

 it possesses all the properties which belong to the pencils into which a ray of 

 ordinary light is divided by a doubly refracting crystal. Accordingly, if such 

 a crystal be placed in the path of such a reflected ray, with the principal plane 

 of the crystal, or a conjugate plane, in the plane of reflection, the ray will not 

 be doubly refracted. But if the crystal be turned in azimuth, two rays will 

 make their appearance, unequal at first in intensity, but becoming equal at the 

 azimuth of 45^. Beyond this azimuth the ray which Avas previously most in- 

 tense fades gradually away, while the other gains in strength, until, at 90^ the 

 former disappears entirely, and the latter remains alone. These phenouKniaare 

 repeated in every quadrant. 



If the ray which has been reflected as above described be incident upon a 

 second surface of glass, at the same angle, (54° 35',) as at first, the plane of 

 second reflection corresponding with that of the first, it is in part reflected and 

 in part transmitted, as is the case with common light ; but if the second plane of 

 reflection be at an azimuth of 90° with that of the first, no reflection at ail will 

 occur, but the whole ray will be transmitted. 



A ray of light therefore, which has undergone the modification which is pro- 

 duced by transmission through a doubly refracting crystal, or by reflection at an 

 incidence of 54° 35' from glass, or at that of 52"^ 45' from water, seems to pos- 

 sess dissimilar physical properties on the sides which arc at right angles to each 

 other, and similar ones on the sides which are diametrically opposed. This 



" The important researches of Kirchhoff iiiid Buusen on the chemical relations of the fixed 

 lines of the spectrum have been published since the preparation of these lectures. 



■j- KuLiiliulf and Bimsen have demonstra.ted that this absorption takes place (at least in case 

 of many of the lines) in the atmosphere of the sun. 



