THE EYE AND VISION. 123 



shall point in opposite directions. How do the parallel 

 lines now look? 



e. Do the long lines appear to be of the same length in 

 Fig. 18? Measure them. 



FIG. 18. 



L. Color-Blindness. 



Material. A large assortment of skeins of worsteds of different 

 colors, shades, and tints. A color-blind person. 



Place a large assortment of the shades and tints of 

 different colored worsteds in a good light. Select a light 

 green skein and ask the person to be examined to select 

 all the skeins of the same color whether lighter or darker. 

 A color-blind person will select shades and tints of other 

 colors. Select a bright red skein and ask the person to 

 point out other skeins of the same color. The red-blind 

 person will select green and gray ; the green-blind person 

 will select red and gray. Do not ask the person to name 

 colors. 



M. Mixing of Color Sensations. 



Apparatus. Rotary apparatus for color-disks. The Bradley 

 Color Wheel and Maxwell Disks. 



On a rotating disk place a blue and a yellow disk in 

 such proportions as to obtain gray or white when rotated. 

 Why does it not make green ? Try different combinations 

 of colors. 



