18 



POLARISATION OF LIGHT. 



plane, when cut parallel to their axis. It 

 has been much used in experiments on po- 

 larisation ; but owing to the colour which 

 it produces, it is of no use whatever in 

 researches where the phenomena of 

 colour are to be studied. When two 

 plates of tourmaline or agate are placed 

 in rectangular positions, not a ray of 

 light is transmitted through them, not 

 even the light of the meridian sun. 



By using any of these pieces of ap- 

 paratus, we can at all times produce 

 either a ray, or a broad beam of polarised 

 light; but when the structure of crys- 

 tallised or organised bodies is examined 

 by observing their affections under 

 polarised light, the light transmitted 

 through their substance requires to be 

 analysed by a reflecting plate, or a 

 doubly refracting prism, or a plate of 

 tourmaline or agate which has the pro- 

 perty of reflecting or transmitting one 

 portion of the polarised, and allowing 

 another portion to be seen which was in 

 a state of combination with the first 

 portion. The use of these analysing 

 plates in prisms will be better under- 

 stood from the following chapter. 



CHAPTER VI. 



Colours produced by the Action of Crys- 

 tallised Bodies upon Polarised Light 

 Systems of Rings produced by 

 Crystals with one Axis of Double Re- 

 fraction Negative System of Rings 

 Positive System of Rings List of 

 Crystals of the Negative and Positive 

 Class Method of calculating the 

 Tints Transformation of the Rings. 



THE phenomena of colour produced by 

 the action of crystallised bodies upon po- 

 larised light, are the most splendid with- 

 in the whole range of optics. The co- 

 lours themselves were first seen in Iceland 

 spar by Huygens, and studied by subse- 

 quent philosophers, but they knew no- 

 thing of their origin or nature. They 

 were discovered, by independent obser- 

 vation, by M. Arago and Dr. Brewster, 

 and the subject has been successfully 

 pursued by these two authors, and also 

 by M. Biot, Dr. Young, M. Fresnel, M. 

 Herschel, and Professor Mitscherlich, 

 from whose labours, to use the words of 

 a distinguished author, " it has acquired 



Fig. 21. 



a developement, placing it among the 

 most important, as well as the most 

 complete and systematic branches of 

 optical knowledge:' In order to exhibit 

 these colours in the simplest manner; 

 let two plates of glass, A, C, Jig. 21, 

 be arranged, as shown in fig. 12, so 

 that the light polarised by the first 

 plate A, (or the polarising plate,} 

 refuses to be reflected by the second plate 

 C, (or the analysing plate.) If the 

 light R A, be that of the sky, which 

 will do very well for ordinary purposes, 



the proper adjustment of the glass plates 

 will be known, by looking at C in the 

 direction E C, and observing a dark 

 undefined spot, in the image of the part 

 of the sky reflected by A. The glass plates 

 should be adjusted till this spot is as dark 

 as possible. In order to increase the 

 quantity of polarised light, and to have a 

 larger surface, it would be desirable to 

 use the bundle of glass plates described 

 in the last chapter, in place of the single 

 plate of glass A. 

 Having placed the polarising plates A, 



