Page:Popular Science Monthly Volume 1.djvu/483

Rh perceptible, either in certain effects of light, or on a peculiar exertion in the straining of the attention. By a simple experiment it is possible to make every person acquainted with these guests of his field of view, the so-called muscæ volitantes; only, one must be prepared for those formerly overlooked, but once honored with attention, never again stirring from their post.

In order, likewise, to preserve space for the play of these variations in the form of the lens, in the act of adjustment, it was necessary to surround the lens with a liquid or elastic medium. That the aqueous humor bears a part in moistening the cornea, has already been stated, but in the voluminous vitreous humor we behold the regulator of the shape and tension of the eye. Such an auxiliary medium is of urgent necessity to keep up the regularity of refraction, the quality of tension in the retina as a sensitive surface, and the action of the optic nerve. Some years ago, I was happy in being able to demonstrate that a comprehensive range of diseased conditions and also blindness, the causes of which had been successively sought for in different parts of the eye, simply arose from too great a tension being exercised by the vitreous humor—a discovery which imparted so much the more pleasure, as a suitable remedy was likewise at hand.

Let us imprint on our memories, from the model of a magnified eye, the positions and dimensions of the different parts. For the present, I begin with the cord of the optic nerve, which, as you perceive, does not enter the sclerotica exactly opposite to the cornea. This part, as well as the larger posterior chamber of the eye, is embedded in the orbit, and is therefore not visible externally; while, on the other hand, between the lids, you remark the white of the eye, which is the anterior chamber of the choroid; next the transparent cornea; and away through it the colored iris, with the pupil in the centre.

The black appearance of the latter used to be ascribed solely to the dark layer formed by the choroid for the interior of the eye, absorbing all the light. More patient and minute investigations have, however, proved that the pupil derives its blackness only partially from the above circumstance, and mostly from the refraction of the light. Helmholtz has succeeded in banishing that darkness from the pupil of the human eye. By a simple arrangement, called the speculum oculi, he uses the light which is reflected from the deeper parts of the eye to illuminate the whole of the interior, as also the image itself projected on the retina. This invention exercises an influence not only on the peculiar branch of the oculist, but likewise on the broad field of medical investigation, seeing it affords an insight into the optic nerve, a direct process of the brain, and other structures, which, along with their analogies, were hidden from observation in the body.

The dimensions of the eyeball among strong-sighted individuals is more equal than you perhaps imagine. The apparent differences of size lie almost exclusively in the shape of the slit of the lid. If it has