DURATION OF STIMULUS AND SENSATION. 1073 



observed by moving the source of stimulation, when the recurrent appears 

 to follow at a certain distance the primary image ; this may be done most 

 readily by means of a revolving mirror. The same phenomenon may also 

 be observed with a momentary stationary stimulus. By the former method 

 some observers have described the recurrent as immediately following the 

 primary image, while others have seen the two separated by a dark 

 interval. This difference has been shown by Charpentier 1 and v. Kries 2 to 

 depend on the intensity of the stimulus, and on the adaptation of the eye. 



Most observers have described the interval as about -V second 

 according to Charpentier it may vary from \ to -^ second with intensity. 

 By altering intensity alone v. Kries did not produce so great a differ- 

 ence, and he regards the length of the interval as chiefly dependent on 

 adaptation. He found that in complete dark-adaptation the recurrent 

 image followed the original immediately, and was brighter than to the 

 light-adapted eye. v. Kries believes, however, that the two appear- 

 ances do not depend simply on shortening of the interval, and he states 

 that, with moderate dark-adaptation, two images may be seen, one follow- 

 ing immediately, the other at an interval. The duration of the stimulus 

 is also of influence ; increased duration shortens the interval, and 

 lengthens the recurrent image (Bosscha). 3 The size of the recurrent 

 image varies with intensity (Charpentier), attaining with strong light 

 five or six times the size of the original. 



The coloration of the image has been differently described. Some 

 have seen it as colourless, whatever the nature of the stimulus ; others 

 as faintly complementary, others as violet or reddish. Hess, 4 on the 

 other hand, describes it as having the same colour as the original, but 

 so faint that it can only be detected by comparing double images. 

 According to Charpentier, it is violet with low, colourless with high, 

 intensity ; while v. Kries describes the ordinary image as faintly com- 

 plementary, the image of the dark-adapted eye as colourless. 



The interval between the original and recurrent image has been 

 described as of abnormal darkness (Bidwell), or as faintly tinged in the 

 complementary colour (Hess). 



When coloured light is used for stimulation, the recurrent image 

 varies with the part of the spectrum. Bidwell 5 and v. Kries have failed 

 to see any recurrent image following spectral red, and according to others 

 it is very faint with this part of the spectrum. The brightest image 

 is obtained by using green light, and, according to Hess, the curve repre- 

 senting the brightness of the image for different parts of the spectrum 

 corresponds closely with the curve of brightness of the spectrum for the 

 dark-adapted eye at low intensity. 



v. Kries has found that the image is not seen in direct vision ; that, 

 as the moving image crosses the region of central vision, it disappears 

 momentarily; and, using the stationary method, he has found that the 

 region of the visual field in which it is absent has a diameter of 

 35 to 38 mm. at 1 metre distance. 6 v. Kries has also found that there 



1 Compt. rend. Acad. d. sc., Paris, 1896, tome cxxii. p. 87. 



2 Ztschr.f. Pyschol. u. Physiol. d. Sinnesorg., Hamburg u. Leipzig, 1896, Bd. xii. S. 81. 

 * Arch.f. Ophth., 1894, Bd. xl. Abth. 1, S. 22. 



4 Arch.f. d. g&s. Physiol., Bonn, 1891, Bd. xlix. S. 190. 



5 Proc. Roy. Soc. London, 1894, vol. Ivi. p. 132. 



6 Hess (Arch. f. Ophth., 1897, Bd. xliv. S. 445) denies this, while others (Samojloff 

 and Hamaker) have confirmed v. Kries' observation. Hess also denies that the influence 

 of adaptation is as described by v. Kries. 



VOL. u. 68 



