Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 2. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
172, 173.] 
§22. Duration of the Sensation of Light 
§22. Duration of the Sensation of Light1 
When a motor nerve is excited by a brief electric shock, a short 
time (about a sixtieth of a second) elapses before the effect of the 
stimulus on the muscle disappears again. The change produced in 
the organic structures as a result of the stimulation lasts much longer, 
therefore, than the electric discharge which causes the reaction. The 
same thing takes place in the eye. We cannot as yet demonstrate, it 
is true, that the sensation arises after the light has begun to act, but 
we can easily show that it continues after the light has quite ceased to 
The more intense the light, and the less fatigued the eye, the longer 
the after-effect will persist. After glancing towards the sun or at a 
bright flame for an instant, and then suddenly closing the eyes and 
covering them with the hand, or looking at an absolutely dark back¬ 
ground, one will continue to see for a short time a bright appearance 
in the dark field, of the same form as the object itself was, which grad¬ 
ually fades away, changing its colour at the same time. The after¬ 
images of very bright objects are easiest to see because they last 
longest. Incidentally, even with less brilliant objects after-images 
such as those described can be obtained, provided the eye has been 
rested sufficiently beforehand in the dark, and the object is then 
observed for a moment. The after-image of a bright body on a dark 
ground has the colour of the object at first, and often shows very ac¬ 
curately still the details of the object in correct form and shading. For 
example, when a person takes a last look at a lamp just in the act of 
extinguishing it and before being left completely in the dark, he 
1 See Appendix B. 7 by W. Nagel at the end of this volume.—N. 
^[See also Appendix II. 4 by v. Kries. (M. D.) 
2 ^fThat there is a demonstrable latent period following the instaht of stimulation, 
before the visual sensation develops, is brought out by Helmholtz a little later in the dis¬ 
cussion of rotating discs. Moreover, that the instant of stimulation of the retina is followed 
by a measurable latent period has been known since the work of Dewar and McKendrick 
(Trans. Roy. Soc. Edinb. 1873. XXVII. 141). They made the first approximate measure¬ 
ments using eyes of birds, mammals, fishes, frogs, lobsters. Einthoven and Jolly (Quart. 
Jour. Exp. Physiol. 1908. I. 373) state that for the frog’s eye the latent period, as indicated 
by the electrical variation following stimulation, varies from 0.01 to 2.0 sec, depending on 
the intensity of the stimulus. They say, also, that these values agree well with the latent 
period of the perception of light by the human eye. Temperature is also a factor, as shown 
by Hecht (Journ. Gen. Physiol. 1919. I. 657-667.) What the latent period of the cerebral 
centres involved may be, we have no way of knowing exactly. 
For recent work on the latent period of the organ of vision consult Einthoven and 
Jolly (loc. cit.); Brücke (Arch. f. d. ges. Physiol. 1906. CXIV. 569); Fröhlich (Zft. f. 
Sinnesphysiol. 1913. XLVIII. 28); Hecht (loc. cit.); Chaffee, Bovie and Hampson 
(Journ. Op. Soc. Amer., 1923, VII. 1); Bills (Psy.Monog. 1920. XXVIII, No. 127, 101) 
(a lengthy paper on the ‘lag’ or inertia, in the rise of a visual sensation to its maximum 
intensity). (M. D.)


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