Volltext: Helmholtz's treatise on physiological optics. Volume 2. Edited by James P. C. Southall. Translated from the 3rd German edition (2)

25, 26.] 
§18. Stimulation by Light 
quite luminous. This is due to the translucent nature of the nervous 
substance. On the retina itself, near the disc, he saw scarcely any trace 
of light that might be due to diffusion in the transparent media of the 
eye or that was reflected sideways from the brightly illuminated sur¬ 
face of the optic nerve. The patient was not conscious of any sensation 
of light as long as the little luminous image fell entirely on the disc. 
Some patients thought they perceived a faint glimmer of light; which 
possibly might be due to the feeble illumination of the retina mentioned 
above. By small movements of the mirror, the image could be made to 
move from one side to the other of the disc, but there was never any 
consciousness of light until a part of the image was distinctly across 
the boundary so as to fall on a place where the various layers of the 
retina were present. This means simply that the blind spot corresponds 
to the entire area where the optic nerve enters, and certainly is not 
just at those places where the blood-vessels enter the eye. 
Subsequently, Coccius1 showed how the same experiment could be 
performed by the observer in his own eye; which is still more instruc¬ 
tive. For this purpose, a plane or convex mirror is used, with a hole in 
it, like the mirror of an ophthalmoscope. The observer holds it close 
in front of his eye, and allows the light from a lamp to come through 
the hole to his eye. If the eye is first turned right towards the edge of 
the hole, the little inveited red image of the flame on the retina of 
one’s own eye can easily be seen. Now trying not to let the image go, 
turn the eye more and more inwards, until presently the image falls 
at the place where the optic nerve is; and then make the observations 
described above. Incidentally, the flame ought to be small or far away; 
otherwise, too much light will enter the eye. In this way the larger 
blood-vessels are seen, but the field, of course, is very small. When a 
larger flame is used, the eye is so blinded by it that it is impossible to 
see much. If the quantity of light falling on the plate where the optic 
nerve enters the eye is large, a faint glimmer of light will certainly be 
perceived, but the explanation of it, according to these experiments, 
is that some of the light spreads out over adjacent parts of the retina. 
Sometimes too in experiments of this kind there is a red glow of light 
in the eye, perhaps when a blood-vessel on the surface of the optic 
nerve is highly illuminated and reflects light. This was seen by 
A. Fick and P. du Bois-Reymond by using for the object the image 
of the sun as made by a convex lens. 
The form and apparent size of the blind spot in one’s own visual 
field may be easily found as follows. Place the eye 8 or 12 inches above 
1 Über Glaukom, Entzündung und die Autopsie mit dem Augenspiegel. Leipzig 1859. 
S. 40, 52.


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