PRACTICAL EXERCISES 997 



(b) Look at the lamp through a coloured glass for thirty or forty 

 seconds, and then close the eye or look at a white ground. The 

 after-image of the filament will appear in the complementary colour 

 of the glass. If the glass was red, for instance, the after-image will 

 be greenish. 



(c) Look at a white square on a dark ground for thirty seconds, 

 then quickly cover the field with white paper. A dark square will 

 be seen on the white ground. 



FIG. 444. APPARATUS FOR COLOUR-MIXING. 



(d) Repeat (c) with coloured squares. The after-image of the 

 square will be in the complementary colour. 



Contrast. Perform Meyer's experiment (p. 944). 



17. Retinal Fatigue. Fix the eye steadily on a portion of a printed 

 page a considerable distance away. Note that the print soon 

 becomes blurred. Wink the eye ; the short rest causes a notable 

 recovery of the retina. 



18. Visual Acuity. Draw on a white card a series of vertical black 

 lines i millimetre thick, and separated from each other by a distance 

 of a millimetre. Set the card up in a good light, and walk back- 

 wards from it till the individual lines just fail to be discriminated. 



