Chemistry and Physics. 393 



Numerous specimens of plate glass and optical glass were 

 examined, and all were found to produce scattering. The kinds 

 of glass, however, differed from one another in the intensity and 

 completeness of polarization impressed upon the transmitted 

 light. In the case of Chance's crown glass the intensity of the 

 vibrations parallel to the direction of the primary beam was 

 experimentally determined to be 8 per cent, of the intensity per- 

 pendicular to this beam. The corresponding ratio for ordinary 

 plate glass was found to be 3 per cent. Strutt says that "In 

 the case of glasses, the wide difference between different samples 

 suggests that scattering is due in the main to inclusions rather 

 than to the molecules. These inclusions are probably to be 

 regarded as spherical, some of them with a diameter not small 

 compared with the wave-length. In this case the defect of 

 polarisation in glass would be due to the appreciable size of the 

 obstacles, whereas in gases it is due to lack of spherical sym- 

 metry. ' ' 



Yellow quartz and smoky quartz were found to have the 

 property of scattering light very strongly, the coloring matter 

 being evidently distributed in the crystal in the form of small 

 particles analogous to those found in glass. Obviously the 

 scattered light should be analyzed parallel and at right angles 

 to one of the principal planes of the crystal in order to avoid 

 the disturbing influence on the relative intensities which other- 

 wise would arise from double refraction. In the case of a crys- 

 tal of yellow quartz from Madagascar the polarization of the 

 scattered light was fairly complete, as the weak (parallel) image 

 had only about 0-7 of one per cent, of the intensity of the strong 

 (perpendicular) one. "This is decidedly more perfect polarisa- 

 tion than was obtained with any of the gases examined in the 

 earlier investigation." The stronger image was bluish, while 

 the fainter was a very rich blue. A specimen of slightly smoky 

 quartz from Brazil gave less scattering than the preceding, the 

 intensity of the weak image being about 3 per cent, of that of 

 the strong image. Nevertheless the color was a ' ' good sky-blue. ' ' 

 The observations above referred to were made when the axes of 

 the primary beam and of the crystal coincided. 



On the other hand, if the line of vision is along the crystallo- 

 graphic axis then the rotatory property of the crystal manifests 

 itself by causing (in white light) the two polarizations to 

 appear to the eye as of equal intensity. The rotatory effect is 

 best shown, however, by using a nicol prism to polarize the inci- 

 dent beam and to allow the cloud of scattering particles to act 

 as analyzer. As the beam advances through the crystal, the 

 plane of polarization is rotated, so that alternations of light and 

 darkness are observed laterally, corresponding to rotations of 

 90°. The colored bands obtained with white light are very 

 striking, but they rapidly lose their purity after a few periods 

 as a consequence of the superposition of different orders. The 



