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
§17. Stimulation of the Organ of Vision 
than the other, he could not see these floating clouds except in his 
right eye. The background of the visual field, on which these phe¬ 
nomena are projected is never entirely black; and alternate fluctuations 
of bright and dark are visible there, frequently occurring inrhythm with 
the movements of respiration; as observed by both J. Muller1 and the 
writer. Moreover, with every movement of the eyes or eyelids, and 
with every change of accommodation, there are accompanying varia¬ 
tions of this “luminous dust.” The shapes that are assumed are very 
curious, especially when one happens to be in a strange place that is 
perfectly dark, as, for instance, in an unlighted hallway where it is 
necessary to grope one’s way; because then these imaginary figures 
are apt to be mistaken for real objects. Under such circumstances 
Purkinje noticed that every unexpected contact and every uncertain 
movement produced instantaneous oscillations of the eye which were 
accompanied by gossamer clouds of fight and other luminous appear¬ 
ances, such as may easily have been the origin of many ghost stories. 
After strenuous exercise and when the body is overheated, Purk¬ 
inje2 noticed a faint glow of fight glimmering in his dark field, like 
the last expiring flickers of a flame of alcohol burning on the top of a 
table. Upon closer examination he detected countless tiny little points 
of fight darting to and fro and leaving little trails of fight behind them. 
He got a similar effect when he closed his right eye and strained to see 
with his other weak eye. 
Another important fact is, that after a person has lost one of his 
eyes, or in case the optic nerves and eyes have degenerated and cease 
to function, he may still have subjective sensations of light.3 Such 
experiences show that not merely the retina, but the trunk and roots 
of the optic nerve in the brain as well, are capable of giving rise to 
sensations of fight as a result of being stimulated. 
Lastly, another powerful agency for stimulating not only the optic 
nerve but all the other nerves of the body is by a current of electricity. 
As a rule, the motor nerves do not produce twitching except at the 
instants when the current traversing them is suddenly increased or 
diminished; but sensations are excited in the sensory nerves not only 
by fluctuations in the current but by a steady flow ; and the quality of 
the sensation depends on the direction of the current. 
When the optic nerve is stimulated by fluctuations in the strength 
of a current of electricity, bright flashes of fight are produced extending 
1 Phantastische Gesichtserscheinungen. S. 16. 
2 Beobachtungen und Versuche, etc. I. 63, 134. II. 115. 
3 See J. Müller, Phantastische Gesichtserscheinungen. S. 30 — A. v. Humboldt, Ge¬ 
reizte Muskel- und Nervenfaser. Tl. II. S. 444. —Lincke, defungo medullari. Lips. 1834.


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