Page:On a Complete Apparatus for the Study of the Properties of Electric Waves.djvu/12

66 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.

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.

The spectrometer circle is removed, and an ordinary stand for mounting the receiver substituted. By fitting the lens-

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.