Fig. 4.
When a ray of light passes from one medium to another, and through that into the first again, if the two refractions be equal, and in opposite directions, no sensible effect will be produced.
REFRACTION, LENSES, FOCUS.
The reader may readily comprehend the phenomena of refraction, by means of light passing through lenses of different curves, by reference to the following diagrams:—
Fig. 5. Fig. 6. Fig. 7.
[Fig. 5] representing a double-convex lens, [Fig. 6] a double-concave, and [Fig. 7] a concavo-convex or meniscus. By these it is seen that a double-convex lens tends to condense the rays of light to a focus, a double-concave to scatter them, and a concavo-convex combines both powers.