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nydus/How it WorksPublic

Archibald Williams explains the scientific principles behind common mechanisms and physical phenomena such as steam, electricity, light, and hydraulics. The text focuses on the fundamental operation of everyday devices rather than specific models or detailed technical specifications.

Page 185 of 446
Table of Contents

HOW IT WORKS.

extremities of the object, a simple ray is considered to pass through the centre of the lens. This is not deflected at all. Two other rays from the same points strike the lens above and below the centre respectively. These are bent inwards and meet the central rays, or come to a focus with them at A1 and B1. In reality a countless number of rays would be transmitted from every point of the object and collected to form the image.

Fig. 108.
Fig. 108. —Showing how an image is cast by a convex lens.

FOCUS.

We must now take special notice of that word heard so often in photographic talk—"focus." What is meant by the focus or focal length of a lens? Well, it merely signifies the distance between the optical centre of the lens and the plane in which the image is formed.

Fig. 109.
Fig. 109.

We must here digress a moment to draw attention to the three simple

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