E. S. Ferry — Persistence of Vision. 195 



minute observations or longer, and found their separate obser- 

 vations differing by a large per cent from the mean. With 

 this apparatus a difference of more than three per cent between 

 two observations of the same region in a spectrum of ordinary 

 brightness is rare. The observations at regions of low bright- 

 ness are more difficult. 



The eye was subjected to the intermittent light for as short 

 a time as possible so as to avoid the secondary colors described 

 bj Signor Cintolesi.* After each observation on colored 

 light the eye was rested by looking at white light and 

 the succeeding observation was invariably made on a dif- 

 ferent color. Thus the disproportionate increase of sensitive- 

 ness of the eye for more refrangible rays due to adaptation, as 

 noticed by M. R. Parinaud,f was guarded against. These ob- 

 servations were made in a room with blackened walls and 

 every attempt was made to exclude extraneous light. It was 

 also soon found that precautions were necessary to eliminate 

 the slight tremor produced by the motor and the disc, because 

 a vibration of the lamp or of the diffraction-grating produces 

 a flicker in the field of view that cannot be distinguished from 

 the appearance produced by a too slow rotation of the sectored 

 disc. 



Duration of Light Impressions on the Normal Retina. 



To represent the normal eye, three persons were selected of 

 about the same age, whose eyes were free from Daltonism, astig- 

 matism, near- and far-sightedness and from such abnormal color 

 sensations as have been recently observed by Captain Abney^: 

 in confirmed users of narcotics and stimulants. How very 

 closely the duration of retinal impression for each part of the 

 spectrum agreed for these cases is shown in the following table : 







Table 



I. 



Wave-length. 



Duration of 



retinal impression in seconds. 



A 



E. S. F. 







G. W. B. E. F. N 



■435 



•0357 







•0357 -0333 



•480 



•0250 







•0263 



•510 



•0200 







•0186 -0200 



•540 



•0156 







•0152 



•570 



•0139 







•0139 '0139 



•589 



•0132 







•0128 -0128 



•615 



•0141 







•0142 



•645 



•0156 







•0152 



•684 



•0192 







•0179 '0192 



* Ann. di Optalmol. II and III, 1879. 



f Del'intensite lumineuse des couleurs spectrales, Comptes Rendus, xcix, p. 937. 

 \ On the Examination for Color of cases of Tobacco Scotoma, and of Abnormal 

 Color-blindness, Roy. Soc. Proa, xlix (1891), p. 491. 



