Lesson 6
FOCUS GLASSES (new term)

Focus-glasses are those which affect and change the focus in our eyes. About ninety-nine per cent of glasses prescribed by oculists are focus-glasses. The lenses have either a plus (convex) curvature which shortens the focus, or a minus (concave) curvature which lengthens the focus. The lensses are either plus, which add curvature to your eyes, or minus, which subtract and take away curvature from your eyes.

It is important, it is very important, that we recognize and emphasize the difference between plus and minus lenses.

They are opposite to each other.
They neutralize each other.
Plus is like a hillMinus is like a hollow
Plus stands for positiveMinus stands for negative
Plus magnifiesMinus reduces
Plus means convexMinus means concave
Plus adds curvatureMinus subtracts curvature
Plus helps nearer seeing    Minus helps farther seeing

The lenses in opera glasses, camera lenses, magnifiers and microscopes are all focus lenses. They all have plus (convex) curvatures which bend or refract light and heat rays which fall on their surfaces. They bend the rays towards an imaginary line from the optical center of the lens to the focus, which is some distance from the lens. This distance depends on the curvature of the lens. A little curvature will have the focus far away. Mor curvature will bring the focus nearer.

Colored glasses and goggles are not focus-glasses because they are either flat or neutral. Prism lenses are not focus lenses because they bend light rays into a line instead of a point or focus. Focus lenses are those which are either plus or minus.

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