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in each section is the corresponding section of the rectangular planes. It will be seen that the beam, at first circular or nearly so, flattens more and more until it becomes a horizontal plane at f; then it becomes more and more elliptical until its section is a small circle at f"; then it flattens horizontally and elongates vertically until it becomes a vertical plane at f'; and then finally it thickens again and at the same time enlarges indefinitely
Application to the Test.-In looking at the test cross, the horizontal line of the cross would be seen perfectly distinctly at f, because there is there no vertical blurring, but only horizontal. This would not affect visibly the horizontal, but would render the vertical line very indistinct. At the distance f', on the contrary, there is no horizontal blurring, but only vertical. The vertical line therefore would be very distinct, and the horizontal indistinct. At f", the two lines are seen equally well, but neither of them quite distinctly. The Remedy.-In a general way we may say
that the defect is remedied by the use of more or less cylindrical lenses adapted to the kind and degree of astigmatism. To take one simplest case: Suppose the horizontal curvature is normal, but the vertical curvature too great: then the glasses must be plane horizontally and concave vertically.
EXPLANATION OF PHENOMENA OF MONOCULAR VISION.
SECTION 1.-STRUCTURE OF THE PETINA.
We have thus far treated of the eye, and compared it with the camera, purely as an optical instrument, contrived to form an image upon a receiving screen suitably placed. We have also treated of the defects of the eye, as much as possible, from the same physical point of view as defects of an instrument. But in both the camera and the eye the image is only a means to accomplish a higher purpose, viz., to make a photographic picture in the one case and to accomplish vision in the other. We have thus far spoken as much as possible only of an insensitive screen, the ground-glass plate in the one case and the dead retina in the other. But in both, when accomplishing their real work, we have a sensitive screen, in which wonderful changes take place, viz., the iodized plate in the one and the living retina in the other. In order to understand the real function of the eye in the living animal, it is necessary that we study the structure and functions of the retina.
Structure of the Retina.-The retina, as already stated, page 15, is a thin membranous expansion of the optic nerve. These nerves, arising from the optic lobes and the thalumus, appear first beneath the base of the brain as the optic roots, rr', Fig. 22, converge, unite, and partially cross their fibers at the optic chiasm, ch ; then, again diverging, enter the conical eye-sockets a little to the interior of their points; then pass through the midst of the fatty cushion behind the eye, surrounded
A VIEW OF THE Two EYEI, WITA OPTIC NERVES.—ch, optic chiasm; rri, nerve
roots ; n and n', right and left optic nerves. (After Helmholtz.)
by the diverging recti muscles, and finally penetrate the sclerotic at a point about one eighth of an inch to the inside of the axes; then spread out all over the interior of the ball as an innermost coat, immediately in contact with the vitreous humor, and extend as far forward as the ciliary processes, or nearly to the iris. The wide extent of this expansion and its hollow concave form are necessary to give wideness to the field of view. By this means rays from objects, not only in front but far to the right and left, above and below, fall upon and impress the retina. The union of the two optic nerves at the chiasm is undoubtedly connected in some way with the wonderful co-ordinate action of the two eyes in every voluntary act of siglit.
The thickness of this nervous expansion is about one hundredth of an inch, or about the thickness of thin cardboard, at the bottom of the concavity where it is thickest, but thins to one half that amount on the anterior margins; yet, under the microscope, a section
GENERALIZED SECTION OF RETINA, ETC.-0. optic nerve; 8, sclerotic; ch, cho
roid; R, retina; b, bacillary layer; g, granular and nuclear layer; s, fibrous layer; V, vitreous humor; C, central spot.
through the thickness shows that it is very complex in its structure, being composed of several very distinct layers. We may first represent it on a smaller scale of enlargement as composed of three principal layers : First, the innermost layer, f, Fig. 23, in contact with the vitreous humor, V, is composed wholly of fine interlaced fibers of the optic nerve. This nerve, o, is seen to pierce the sclerotic and the other layers of the retina, and then to spread out as an innermost layer. This is
is called the fibrous layer. Second, outermost of all, and therefore in contact with the choroid, ch, is a remarkable layer, composed of cylindrical rods, like: pen
ENLARGED SECTION OF RETINA (after Schultze). — A, general view; B, nervous
elements; a, bacillary layer; b, interior limit of this layer; c, external nuclear luyer; d, external granular layer; e, internal nuclear layer: f, internal granular layer; g, ganglionic layer; h, fibrous layer, consisting of fibers of optic nerve.