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
18, 19.] 
§17. Stimulation of the Organ of Vision 
with naïve suppositions as to how images of objects can possibly reach the 
mind. Democritus and Epicurus believed that the images were let loose from 
the objects and flew into the eye. Empedocles made the rays proceed to the 
object not only from the source, but from the eye also, and argued that the 
object was thus, so to speak, touched by the eye. Plato’s opinions vacil¬ 
lated. In the Timaeus he accepts the views of Empedocles: the rays issuing 
from the eye are like rays of light except that they are without heat, and the 
only way that vision occurs is when the internal light from the eye proceeds 
to the object and encounters the external light. On the other hand, in the 
Theaetetus, his reflections as to the spiritual basis of the perceptions lead him 
to entertain views that are not very far apart from the more mature stand¬ 
point of Aristotle. 
Aristotle1 made a delicate psychological analysis of the part played by 
the spiritual reality in the sense-perceptions. Physically and physiologically, 
sensation is clearly different from what it is psychically; and the perception of 
external objects does not depend on some kind of delicate tactile feelers 
emanating from the eye (such as Empedocles’s nerves of vision), but is due 
to an act of judgment. Physically, indeed, his ideas are very undeveloped, 
but in the fundamental conceptions the germ of the undulatory theory can be 
traced. For according to Aristotle, light is nothing corporeal, but an activ¬ 
ity (hepyeta) of the intervening transparent medium, which when at rest 
constitutes darkness. However, he still does not abandon the notion that the 
effect of light on the eye is not necessarily of the same nature as that of the 
luminous source by which it is excited. He tries rather to account for this 
correspondence between cause and effect by the fact that the eye also con¬ 
tains transparent substances, which may be put in the same state of activity 
as the external transparent medium. 
Aristotle’s peculiar and striking contributions to the theory of vision 
passed without notice during the middle ages. Francis Bacon and his 
successors were the first to take up these threads again in their keen dis¬ 
cussions of the connection between ideas and sensations: until Kant in his 
Critique of Pure Reason put an end to their theory. 
At this same time natural philosophers were interested only on the 
physical side of the theory of vision, which had developed rapidly from the 
time of Kepler. Haller formulated the general theory of nerve excitability; 
and described quite clearly and correctly the relation between light and 
sensation and between sensation and perception.2 But more exact knowledge 
concerning excitation of the eye by other stimuli was still lacking; or at least 
what was known was fragmentary and regarded as simply curious. To 
Goethe belongs the credit of having brought the importance of this know¬ 
ledge to the attention of German scientists; although he did not succeed in 
winning them over to a revised theory of the physics of light from the stand¬ 
point of the direct visual sensations, which was the real purpose of his famous 
treatise on Colour Theory. Soon after came the important observations of 
Ritter and other electrical workers concerning excitations of the sensory 
nerves; and above all, Purkinje’s observations; so that in 1826 J. Müller 
could state the chief laws of the subject in his Theory of Specific Sense Energy 
as it was first published in his work on the Comparative Physiology of Vision, 
to which reference was made at the beginning of this chapter. This work and 
that of Purkinje are closely related to Goethe’s Colour Theory, although 
J. Müller subsequently abandoned its physical concepts. Müller’s law of 
specific energies was a step forward of the greatest importance for the whole 
1 De sensibus, de anima lib. II. c. 5-8 and de coloribus. 
2 Eiern. Physiolog. Tom. V lib. 16, 17.


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