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73. A plane mirror half the height of an object gives an image which embraces the whole height. This is readily deduced from what has gone before. 74. If a plane mirror be caused to move parallel with itself, the motion of an image in the mirror moves with twice its rapidity. 75. The same is true of a rotating mirror: when a plane mirror is caused to rotate, the angle described by the image is twice that described by the mirror. 76. In a mirror inclined at an angle of 45 degrees to the horizon, the image of an erect object appears horizontal, while the image of a horizontal object appears erect. 77. An object placed between two mirrors enclosing an angle yields a number of images depending upon the angle enclosed by the mirrors. The smaller the angle, the greater is the number of images. To find the number of images, divide 360 by the number of degrees in the angle enclosed by the two mirrors, the quotient, if a whole number, will be the number of images, plus one, or it will include the images and the object. The construction of the kaleidoscope depends on this. 78. When the angle becomes 0 in other words, when the mirrors are parallel the number of images is infinite. Practically, however, ve see between parallel mirrors a long succession of images, which become gradually feebler, and finally cease to be sensible to the eye. Reflection from Curved Surfaces: Concave Mirrors. 79. It has been already stated and illustrated that light moves in straight lines, which receive the name of rays. Such rays may be either divergent, parallel, or convergent. 80. Rays -issuing from terrestrial points are necessarily divergent. Rays from the sun or stars are, in consequenceof theimmense distances of these objects, sensibly parallel. 81. By suitably reflecting them, ...
