On a visible stellation of the normal and of the cataractous crystalline lens of the human eye / by John Tweedy.

  • John Tweedy
Date:
[1874]
Catalogue details

Licence: Public Domain Mark

Credit: On a visible stellation of the normal and of the cataractous crystalline lens of the human eye / by John Tweedy. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    point of light at a distance for which the eye is not accommo- dated. We know, also, that by passing a small point of light into the eye, we may observe the rays proceeding from a point of light to pass into the well known radiating entoptic image. The monophtlialmic polyopia, rays of stars, and radiating lines of light in the entoptic images are, therefore, all dependent on the same peculiarity in the structure of the lens, namely, the existence of the Stella. Donders has, moreover, pointed out the fact that the sectors of the lens which are divided off by the rays of the stella form distinct images of a point of light, for by moving a small aperture before the pupil, he has observed that when the aperture is opposite a sector, a single image is formed; but when it is moved to the boundary between two sectors, two faint images appear, of which, on further displacement, the one first seen disappears. On rapidly moving the small aperture before the pupil, the little images of light jump as it were from one sector to another.* We may also determine subjectively the influence of the rays of the stella in vision by regarding a large number of fine concentric and equidistant circles, such, for instance, as those figured in Helmholtz plates.*!* In that figure the inner circle is about the fiftieth of an inch in diameter, and the outer one about 2 inches in diameter, the intervals between the peripheries of the intervening circles being about the fiftieth of an inch. If the eye be made myopic by placing- before it a convex glass of, say, 6 inches focal length, and the figure placed at about 5 inches in front of the eye, all the fine circles will be seen clearly defined, that is, if the eye of the observer be otherwise emmetropic. At about 5T% inches the small inner circle becomes obscured on one side, and at about the fifth of an inch further, a second small circle appears at the obscured side. Further still, the circle becomes crenated by the multiplication of images, passing through many sectors. At about 6 inches the circles will no longer be clearly seen throughout their whole circumference, but become divided into sectors, which at 6 do inches are dis-
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