712 THE SENSES 



21. Duration of the Retinal Image. When a beam of light falls 

 upon the retina for an instant it produces a stimulus which endures for a 

 time after the stimulus is removed. This interval can be measured by the 

 proper mechanical device. Place on the color wheel a disc, which has a 

 small segment cut out at one point on the periphery. Put a printed page 

 behind the segment with the observer standing in front. Rotate the segment 

 faster and faster until the printed page is seen continuously. At this point 

 the visual image made at one revolution of the disc lasts until the next im- 

 pression on the same spot. The speed of the revolution of the color wheel 

 can be measured by attaching an electric contact key and signal magnet to 

 the disc wheel and measuring the rate of interruptions against the known 

 vibrations of a tuning fork. The same phenomenon may be determined by 

 placing on the disc 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 color 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 



