194 E. S. Ferry — Persistence of Vision. 



different intensities ; to compare these curves with another 

 showing the distribution of luminosity in the spectrum used ; 

 and finally to compare similar curves obtained from dichroic 

 eyes. 



The apparatus for the measurement of the retinal impression 

 consisted of a diffraction-grating spectrometer ; a sectored disc 

 that could be revolved by an electric motor, interposed between 

 the lamp and collimator; and a chronograph to register accu- 

 rately the number of revolutions of the disc. The source of 

 light was a hundred-volt Edison incandescent lamp supplied 

 by a secondary battery, and it was kept at constant candle 

 power by varying the resistance in circuit so that a volt-meter 

 would always indicate one hundred volts. In front of the 

 lamp was mounted a large condensing lens for the purpose of 

 projecting upon the collimator slit an enlarged image of the 

 filament. In this way a uniform distribution of light was ob- 

 tained in all parts of the field of the spectrometer. At the 

 focus of the eye-piece of the telescope was placed a diaphragm 

 so as to isolate from the spectrum the single color it was de- 

 sired to observe. The disc had a ninety-degree sector cut out 

 from each end of a diameter so that when the disc revolved 

 there would be seen in the telescope equal periods of illumina- 

 tion and of darkness. The speed of the disc was controlled by 

 means of a friction-brake managed by the observer. On the 

 shaft of the disc was mounted a contact device by means of 

 which an electric circuit was made for an instant on each revo- 

 lution of the disc. This current was conducted to the primary 

 of an induction coil having the secondary so connected to a 

 chronograph cylinder that a spark would puncture a blackened 

 paper on the cylinder every time the sectored disc revolved. 

 Pressing against the chronograph cylinder was a stylus electric- 

 ally connected to the escapement of a standard clock. By this 

 device the number of revolutions of the sectored disc in a 

 second could be very accurately determined. 



When taking an observation, the experimenter sits at the 

 telescope of the spectrometer with one hand on the brake 

 regulating the speed of the sectored disc. The disc is first 

 made to revolve so slowly that the field of view in the tele- 

 scope flickers, and then the speed is gradually increased till the 

 point is reached when the field just becomes quiet ; then a key 

 is pressed and an assistant rotates the chronograph cylinder for 

 five seconds and takes the record of the speed of the sectored 

 disc. This gives the duration of the maximum impression on 

 the retina. Such a short time of observation as here used has 

 many manifest advantages. Even when the disc is steadied 

 by a heavy fly-wheel as was done in these experiments, and 

 rotated by a powerful motor, the speed can be kept constant 

 for only a short time. Other experimenters have taken one- 



