]8U.] New Properties of Light. 195 



V. Polarisation of Light by bundles of Glass Plates. 



As a brief abstract of these experiments was given in the last 

 Number of the ylnnals of Philosophy, it is unnecessary to repeat 

 them here. In my paper on this subject I have determined from 

 numerous experiments the law by which the phenomena are regu- 

 lated ; and have pointed out the application of them in explaining 

 the polarising power of doubly refracting crystals. A bundle of 

 glass plates is, in fact, an artificial crystal that forms a single 

 polarised image ; and when the bundle is composed of a particular 

 number of plates, it gives a bright and a neljulous image polai'ised 

 in opposite planes, like the two images of the agate. By coml)ining 

 bundles of plates at different angles and in different planes, a 

 mimber of curious results may be obtained, and a ray of light may 

 be polarised in whatever direction it is incident upon the compound 

 bundle. The most important result, liowever, is the power of a 

 bundle of plates to polarise a beam of light at any angle of inci- 

 dence above the iniuimmn polarising angle. 



VI. Optical Properties of Alcohol of Sulphur. 



This singular fluid has the same relation to all other fluids as tha 

 diamond lias to the precious stones. It possesses a greater refractive 

 potver than any fluid that is at present known ; and a greater 

 dispersive power than any of them, except oil of cassia. In its 

 power of dispersion it ranks between balsam of Tolu and phos- 



d R 

 phorus. The value of RT~i ? or the dispersive power, is 0-115. 



Its refractive power is about I'GSO : but I do not give this result as 

 correct, as I have not yet ascertained by what coloured ray the 

 •pectrum is bisected. 



VII. Optical Properties of Nitrate of Potash. 



This salt possesses the most remarkable optical properties of any 

 substance that is at present known. Its various actions upon light 

 are of the most anomalous and instructive nature. 



1. It greatly exceeds calcareous spar in its double refraction, the 

 difference between the two indexes of refraction in the spar being 

 0*146", and in nitrate of potash 0-180. 



2. The least refracted image is nebulous ; and, what will scarcely 

 be credited, it is formed by a refractive poirer the same us thai of 

 u(iter. The tv.o images are polarised like those of all other doubly 

 rtfracting crystals. 



3. The dispersive power of tiie bright image is rernarkably great, 

 exceeding even that of flint glass ; wiiile the dispersive power of the 

 nebulous image is extremely small. Tlie difference indeed is so 

 striking, that we have here an oailar demonstration of the existence 

 of two dispersive powers in the same crystal, which 1 had formerly 

 fbtaliliihel by luimeroiis experiments. 



N. ii 



