Читать книгу A Practical Manual of the Collodion Process. Giving in Detail a Method For Producing Positive and Negative Pictures on Glass and Paper онлайн

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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.

ssss1 representing a double-convex lens, ssss1 a double-concave, and ssss1 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.

ENLARGING OR REDUCING IN COPYING.

Fig. 8.

Fig. 9.

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These remarks have been introduced here as being important for those who may not understand the principles of enlarging or reducing pictures in copying.

LENSES.

I would remark that the points F and A, in Fig. 9, are termed "conjugate foci."

If we hold a double-convex lens opposite any object, we find that an inverted image of that object will be formed on a paper held behind it. To illustrate this more clearly, I will refer to the following wood-cut:—