If we take into account the electric radiation only, it would no doubt be found that radiations having different wave-lengths are unequally absorbed by different substances.
Phenomena of Interference.
Determination of the Wave-Lengths by Diffraction Gratings.—In a paper read before the Royal Society in June last (vide Proceedings of the Royal Society, vol. lx.) I have given an account of a method of obtaining pure spectra of electric radiation by means of curved gratings. The experiment was carried out with a large apparatus. The spectrum obtained was well defined, and appeared to be linear, and not continuous. I had not time to adapt the experiment to this small apparatus, but I think it would not be difficult to do so.
Double Refraction and Polarization.
The spectrometer circle is removed, and an ordinary stand for mounting the receiver substituted. By fitting the lens-
Fig. 8.—Polarization Apparatus.
K, the Crystal-Holder. S, a Piece of Stratified Rock. C, a Crystal. J, the Jute Polarizer. W, the Wire-Grating Polarizer. D, the Vertical Graduated Disk, by which the Rotation is measured.
tube the electric beam is made parallel. At the end of the lens-tube there is a slot in which is dropped the wire-grating polarizer. A crystal-holder provided with three sliding jaws is fitted on to the lens-tube, and is capable of rotation round an axis parallel to the direction of the electric ray.
