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THREE-DlMENSIOISrAL MOTION PICTURES 53 ble star and its position is read on the scale of the disc which corresponds with the optical axis of the telescope. A vessel filled with mercury is placed at a suitable distance from the tele scope. From its former position, the telescope is aimed at the image of the star reflected by the mercury. A reading is also taken on the graduated disc and it will be seen that the two lines drawn from the center of the axis of revolution of the tele scope to the readings obtained from two equal angles with the horizontal passing through the same center. An elementary geometrical construction proves that the angle made by the light rays of the star with the normal to the mercury surface is equal to the angle made by the reflected light with the same normal. The second law is proved by the arrangement of the appa ratus itself. INTENSITY OF REFLECTED LIGHT It "is quite obvious that, due to the partial absorption and scattering of the light propagated by the luminous body, the intensity of the reflected light is less than the intensity of the incident light. The intensity of the reflected light is also dependent upon the nature of the reflecting surface, its degree of smoothness and upon the obliquity of the incident ray. The power of absorbing light varies with different substances and this influences the visual brightness of each substance as well as the intensity of the light reflected by them. The highly polished surface of a silver mirror reflects more light than the equally smooth surface of a sheet of paper. If a reflecting surface is very coarse, it presents to the inci dent light a great number of small surfaces under an infinite variety of angles and consequently the reflected light is so divided by reflection in so many directions that its intensity is much less than the intensity of the light which would be re flected by a highly polished surface of the same substance.