Volltext: Elements of Physiology

power of these two refracting media may perhaps be disproportionate 
to their refractive power, and this may the source of the achromatic 
property of the eye. 
d. Chromatic property of the eye.—It is an error to regard the 
human eye as perfectly achromatic. Whenever the image does not 
fall at its proper focal distance upon the retina, a more or less distinct 
appearance of colours is produced. The dioptric fringes of colours pro¬ 
duced in our eye by its refractive media, and capable of being excited 
to a certain extent at will, appear to have been first observed by 
Scheiner. The following are the results of my own observations: 
1. If we look upon a white space on a black ground with one eye, adapting the 
refractive power of the eye to a more distant point than the object, the latter—that is, 
the white spot—will appear indistinct, and as if surrounded with a delicate and nar¬ 
row fringe of colours passing from the white to the black ground through violet, 
blue, yellow, and red. Generally the blue and yellow have alone any degree of 
2. If we look upon the same object, and then adapt the refractive power of our eye 
to the perception of an imaginary object nearer to the eye, the coloured fringe of the 
image will present the same succession of colours—red, yellow, blue, and violet, but 
in the reverse order; the violet and blue are now nearer to the black, and the red and 
yellow next to the white. 
When the vision of an object is rendered indistinct while both eyes are open, 
and two images are consequently seen, the colours in the surrounding fringes 
have the same arrangement as in the first case just described, if the axes of the 
eyes are made to converge to a point behind the object; wiiile their order of suc¬ 
cession is reversed as in the second case, if the axes of the eyes meet in front of 
the object. 
The frames of the window also appeared fringed with very vivid colours when we 
gaze at distant objects through it; or if, while looking at the window, we fix our eyes 
upon a nearer object, as the finger held before them. 
Many persons complain of seeing coloured fringes while their power 
of vision is otherwise perfect,—when there is no tendency to morbid 
changes in the retina, or to amblyopia or amaurosis. We have an in¬ 
stance of this phenomenon also in the red borders which surround the 
letters of print, when the adjusting power of the eyes is paralysed as'a 
consequence of passion, of mental exertion, or inclination to sleep, &c. 
The coloured fringes are very evident also when the faculty of the eye 
to accommodate itself to distinct vision at different distances is suspended 
by the action of belladonna. (See my work, Zur Physiol, des Gesichts¬ 
The dioptric fringes of colours arising from refraction within the eye 
must not be confounded with the real coloured halos of luminous objects. 
All the phenomena investigated in the preceding chapter are expli¬ 
cable by reference to the structure of the eye as an optical instrument, 
—that is, by the form and arrangement of the transparent media in 
front of the retina. There are a great number of other phenomena, 
however, of which the structure of these parts affords no explanation, 
b ut which are the results of vital properties of the retina, and of the 


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