80 HISTORY OF OPTICS. 



which we have spoken) continued to occur for a considerable length 

 of the ray. But other persons, attempting to repeat his experiments, 

 confounded with them extraneous phenomena of other kinds; as the 

 Due de Chaulnes, who spread muslin before his mirror, 6 and Dr. 

 Herschel, who scattered hair-powder before his. 7 The colors produced 

 by the muslin were those belonging to shadows of gratings, after- 

 wards examined more successfully by Fraunhofer, when in possession 

 of the theory. We may mention here also the colors which appear 

 on finely-striated surfaces, and on mother-of-pearl, feathers, and similar 

 substances. These had been examined by various persons (as Boyle, 

 Mazeas, Lord Brougham), but could still, at this period, be only looker] 

 upon as insulated and lawless facts. 



CHAPTER IX. 

 DISCOVERY OF THE LAWS OF PHENOMENA OF DIPOLARIZED LIGHT. 



BESIDES the above-mentioned perplexing cases of colors produced 

 by common light, cases of periodical colors produced by polar- 

 ized lirjlii began to be discovered, and soon became numerous. In 

 August, 1811, M. Arago communicated to the Institute of France an 

 account of colors seen by passing polarized light through mica, and 

 analysing' 1 it with a prism of Iceland spar. It is remarkable that the 

 light which produced the colors in this case was the light polarized by 

 the sky, a cause of polarization not previously known. The effect 

 which the mica thus produced was termed depolarization; not a very 

 happy term, since the effect is not the destruction of the polarization, 

 but the combination of a new polarizing influence with the former. 

 The word dipolarization, Avhich has since been proposed, is a much 

 more appropriate expression. Several other curious phenomena of the 

 same kind were observed in quartz, and in flint-glass. M. Arago was 

 not able to reduce these phenomena to laws, but he had a full convic- 

 tion of their value, and ventures to class them with the great steps in 



c At. Par. 1755. 7 Phil. Trans. 1807. 



1 The prism of Iceland spar produces the colors by separating the transmit- 

 ted rays according to the laws of double refraction. Hence it is said to ana- 

 lyse the lisht. 



