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tfftinhiaii laiuval ®h$mtx Vol. X, APRIL, 1908. No Sew Series. Some Optical Phenomena, By Miss Graham Hagerty, A.R.C.Sc. In the dim dawn of the Earth's history, while as yet it was without form, the com¬ mand was given, " Let there be Light." It is to light chiefly that the astronomer owes his knowledge of the heavenly bodies; and I pro¬ pose here to deal with some optical phenomena of more or less common occur¬ rence. It is now generally accepted ■that light has its origin in the vibrations of the molecules of the luminous body, and is transmitted by undulations in an invisible medium—the ether. The ether is supposed to fill all space, to interpenetrate all matter, and to lie between the molecules of even the densest solid. The undulations transmitted are transversal. In the original conception of the undulatory theory it was thought that the particles of ether underwent transversal vibrations, but, if the particles set each other in motion by their mutual attrac¬ tion, the forces would be inclined to the direction of motion of the wave, and there would be a tendency for each particle to be set in motion longitudinally as well as trans-