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Sphere with arcs and circles labeled.

An illustration of a sphere showing diameter, arcs, and circles. A sphere is a solid bounded by a curved…

Sphere With Arcs, Circles, and Zones Labeled.

An illustration of a sphere showing diameter, arcs, and circles. A sphere is a solid bounded by a curved…

Perpendicular Line Drawn From Point P to Line RS

A perpendicular line drawn from point P to straight line RS.

Right Triangle For Finding Distance Across a River

Right triangle ABC with angles A, B, C to be used for finding distance across a river. This is a trigonometry…

Circle With Center o and Radius r with point P

Circle modeling the earth. O is the center of the earth, r the radius of the earth, and h the height…

Curve in Pavement of Road

Illustration of blueprint used by highway engineers to widen the pavement on the inside of the curve…

Coordinate Axis With Angles, Lines, and Perpendiculars Drawn

Coordinate axis with angle XOP equal to theta, Θ, and angle XOQ=180 - Θ. From any point in the terminal…

Coordinate Axis With Perpendiculars Drawn To Form Similar Right Triangles

Angle XOP=Θ and angle XOQ=90+Θ. From a point in the terminal side of each a perpendicular line is…

Coordinate Axis With Perpendiculars Drawn To Form Similar Right Triangles From Positive and Negative Theta, Θ

Angle XOP=Θ and angle XOQ=- Θ. From a point in the terminal side of each a perpendicular line is drawn…

Projection of Points in Circular Motion

Illustration of the projection of point P as it moves around a vertical circle of radius 3 in. in a…

Projection of Points in Circular Motion

Illustration of the projection of point P as it moves around a vertical circle of radius 2 ft. in a…

Resultant Vectors

Illustration of the resultant vector when two vectors are acting upon a body at point P.

Resultant Vector From 4 Forces

Illustration of the resultant vector when four vectors are acting upon a body at point P.

Resultant Vector With Vectors at 90 degrees

Illustration of the resultant vector when two vectors are acting upon a body at point P at 90 degrees.

Resultant Vector With Vectors at Angle Ǝ

Illustration of the resultant vector when two vectors are acting upon a body at point P at Ǝ degrees.

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