Section III., 1908. 



f 195] 



Tranb. R. p. C. 



XIX. — Some Phenomena of the Persistence of Vision} 

 By Professor Frank Allen, Ph.D. 



Communicated by Professor H. T. Barnes, D.Sc. 



(Read xMay 28th, 1908). 



The experiments which are discussed in this paper were suggested by 

 some described in a communication ^ to the Physical Review some yeans 

 ago. A full description was there given of the method of measuring 

 the persistence of vision, which was originally devised by E. L. Nichols. 

 The essential features of this method are few and simple. In front of 

 the slitrof the spectrometer is placed a sectored disk, which, when 



Fig. 1. 



rotated by an electric motor, interrupts the light, causing a flickering of 

 the part of the spectrum under observation. By electrical means, the 

 speed, of the disk is recorded on paper on a rotating drfum, which enables 

 the duration of a single flash of light upon the eye to be determined 

 when the speed of rotation of the disk is such that the flickering just 

 disappears. The duration of those light impulses varies with the lumin- 



' Frank Allen, Phys. Rev., Vol. XL, 1900, p. 257. 

 Sec. III., 1908. 13. 



