A further study of the so-called horopter, making ocular rotations easy of understanding / G.C. Savage.

  • G. C. Savage
Date:
1907
Catalogue details

Licence: In copyright

Credit: A further study of the so-called horopter, making ocular rotations easy of understanding / G.C. Savage. 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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    fore, it is not measured by either the half or the whole of the inter- vening retinal are. A glance at Figure 2 will show this. It is true that the angle B-n-A in this faulty figure, being an inscribed angle, is measured by half the arc B-A, and the angle c-n-b is equal to the angle B-n-A, for they are opposite angles, but it is not true that half the arc B-A is equal to the whole retinal arc c-b. If we must measure the distance of retinal points from the fovea by milli- meters, we should measure the distance between spacial points by meters and not by degrees. Most authors who have not rejected Helmholtz’s axial-ray theory have been consistent in that they have measured corresponding retinal points in millimeters from the fovea, but have been inconsistent in that they have measured the dis- tance between spacial points in degrees. Helmholtz was wrong in teaching that axial rays are lines of di- rection, and this error grew out of his first and greater error, the incorrect location of the poles of the eye. These errors are alike in that he began at the cornea and worked backward to the retina. He should have started at the retina and then worked forward. It must be universally conceded that the retinal meridians all cross each other at the fovea, nor can it be denied that this point of cross- ing must be the posterior pole of the eye. How easy now to construct the optic axis by extending a line from the posterior pole through the center of retinal curvature (the center of rotation) to the cornea, and thus find the anterior pole, a course directly opposite the one pursued by Helmholtz. The anterior pole thus located is usually to the nasal side of the center of the corneal curve, but in ideal eyes it coincides with the center of the corneal curve. In all eyes the posterior pole is the fovea centralis, but the anterior pole may or may not lie the center of the corneal curve. (The real anterior pole is in the aqueous chamber, just as far in front of the center of rotation as the fovea is behind it.) In every eye the antero-posterior axis, or optic axis, prolonged is the visual axis, as distinguished from all other visual lines. This line is a radius of retinal curvature pro- longed, and so are all other lines of vision. Otherwise an isogonal spacial circle with corresponding retinal arcs could not be con- structed. The true law of direction, therefore, must be this: “Every line of direction is a radius of retinal curvature prolonged.”
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