THE SENSE ORGANS 201 



Close the eyes for a few seconds. Look at a gas flame in which some 

 common salt is burning, 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 After-images. 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 square 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 ROTATION BY COMBINING 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 Maxwell'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 respectively 

 red and green, and adjust the colored areas so as to obtain a dark yel- 



