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Lines for the detection of astigmatism. "The refracting surfaces of the eye acting together are equivalent in refracting power to a single, spherical surface of fairly short curvature. Frequently, however, the result is not the same as would be given by a perfect spherical surface, owing to inequalities in the curvature of the eye. In one direction the curvature may be greater than that at right angles to it. This tendency to a cylindrical form is called astigmatism. It interferes with the formation of perfect images and sometimes leads to serious eye strain in the effort to better the vision. Astigmatism may be detected by looking at black lines radiating from a point or at fine black concentric circles. Portions of the liens or circles appear gray and others black; the gray portions are out of focus. This defect is corrected by proper cylindrical glasses which equalize the curvature of the eye" — Newell, 1900

Detection of Astigmatism

Lines for the detection of astigmatism. "The refracting surfaces of the eye acting together are equivalent…

The right retina as it would be seen if the front part of the eyeball with the lens and vitreous humor were removed. The white disk to the right marks the entry of the optic nerve (blind spot); the lines radiating from this are the retinal arteries and veins. The small central dark patch is the yellow spot, the region of most acute vision.

Retina Blind Spot

The right retina as it would be seen if the front part of the eyeball with the lens and vitreous humor…