LIMITS OF THE FIELD OF VISION 677 



two complemental colors and judging the speed of revolution required for 

 complete fusion. 



22. Limits of the Field of Vision. The limits of the visual field are 

 determined by direct measurement with the perimeter. Set the person whose 

 retina is to be measured in a comfortable erect position, with one eye at the 

 center of the arc of the perimeter and the other covered by an eye-shade. 

 The observed eye must be fixed on the center of the field of vision, and care 

 must be used to prevent obstruction of the field. The examination is made 

 with greatest accuracy by bringing an object into the field of vision from behind 

 the person observed. When the individual examined first detects the presence 

 of the object, he announces it and the angle is read off from the arc of the 

 perimeter and recorded on the chart for the purpose. These readings should 

 be made in about twelve radii. They should be made for each eye. 



23. Limits for the Field of Vision for Color. To measure the limits 

 of the field of vision for colors one should proceed as in the preceding experi- 

 ment, except that small squares of colored papers are brought into the field 

 from the rear. The retina should be mapped for red, green, yellow, and blue. 

 Use Bradley's pure color papers. Take four penholders and mount on the 

 end of one a centimeter square of red paper, on the others green, yellow, and 

 blue. To make a determination bring the color up from behind and, as soon 

 as it is certainly detected and announced, remove it from the field of vision. 

 Examine the eye for all four colors at one sitting, mixing them indeterminately 

 in the individual tests. Occasionally an eye will be found which exhibits a 

 well-marked restriction of the color field, though the individual himself may 

 not be completely color-blind. 



24. Color-Blindness. Make an examination for color-blindness, 

 using Holmgren's colored yarns. Spread the yarns out on a table in the best 

 of light. Place the three confusion skeins in front of the individual to be ex- 

 amined and ask him to match them quickly from the skeins on the table, 

 paying no attention to lights and shades of the same color. A color-blind 

 individual will confuse colored skeins, most usually the reds, greens, and grays. 



25. Color Mixing. Use Bradley's color wheel and test the effect 

 of simultaneous stimulation of the retina with two or more colors, by placing 

 on the wheel two or more colored discs, rotating the wheel at a speed sufficient 

 to cause complete fusion. The sensation produced by two colors applied 

 simultaneously will be entirely different from that produced by either alone. 

 Red and green (or greenish blue), when mixed in the proper proportion, pro- 

 duce a sensation of gray. The same effect may be had from yellow and blue, 

 orange and violet, or any of the complementary colors chosen according to 

 the geometrical color table, figure 473. By mixing three colors, red, green, 

 and violet, in the proper proportion one can produce a sensation almost the 

 same as that produced by white light. 



26. Color After-images. Color after-images can be demonstrated 



