. Advanced lessons in practical physiology for students of medicine. Physiology. THE SENSE OUOANS 201 Close tho pyos for a fow seconds. Look at a fias flame in which some common salt is burninji;, and then at a sheet of white paper. Does the after-image move with the eye? Obtain an after-image by suddenly turning off the gas in a dark room, or by gazing at a window for half a second and then closing the eyes. 8. Negative Having rested the eyes, gaze for about half a minute at a small white square on a black background. Suddenly place a sheet of white paper over the black. How do
. Advanced lessons in practical physiology for students of medicine. Physiology. THE SENSE OUOANS 201 Close tho pyos for a fow seconds. Look at a fias flame in which some common salt is burninji;, and then at a sheet of white paper. Does the after-image move with the eye? Obtain an after-image by suddenly turning off the gas in a dark room, or by gazing at a window for half a second and then closing the eyes. 8. Negative Having rested the eyes, gaze for about half a minute at a small white square on a black background. Suddenly place a sheet of white paper over the black. How does the present after-image differ from those obtained previously. Look at a'black sciuare on a white ground; then suddenly let the eyes rest upon a gray screen. Observe the after-image, and note the apparent increase in its size when the gray screen is moved farther away from the eye. Gaze at a bright red square upon a black ground; then look at a white surface. Does the bluish-green after-image show periodic varia-. FiG. 115.—Rothe's Rotatory Apparatus for Color Disks. It is so Arranged as to Give Various Rates of Rot.\tion by the Motions of 1, 2, and 3. tions? Repeat the experiment with a green square. Note that the color of the after-image is complementary to that of the square. Place the red and green squares side by side on the black surface. Look at them steadily for about twenty seconds, and then slip a sheet of white paper over the whole. 9. Fusion of Gray and White.—By means of jMaxwell's color mixer rotate the black and white disk supplied for this experiment until you have obtained complete fusion of the black and white in the outer zones of the disk. Increase the frequency of the revolutions of the disk until even the central zones appear of a uniform gray color. Darken the room some- what, and determine how many revolutions are necessary to produce a gray of the same quality as before. 10. Fusion of Colors.—Rotate two large disks, colored respectiv
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