d photographic plate 1,000
times more quickly.

[Illustration: FIG. 107.]

THE IMAGE CAST BY A CONVEX LENS.

Fig. 108 shows diagrammatically how a convex lens forms an image. From A
and B, the 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 A^1 and B^1. In reality a countless number
of rays would be transmitted from every point of the object and
collected to form the image.

[Illustration: 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.

[Illustration: FIG. 109.]

We must here digress a moment to draw attention to the three simple
diagrams of Fig. 109. The object, O, in each case is assumed to be to
the right of the lens. In the topmost diagram the object is so far away
from the lens that all rays coming from a single point in it are
practically parallel. These converge to a focus at F. If the distance
between F and the centre of the lens is six inches, we say that the
lens has a six-inch focal length. The focal length of a lens is judged
by the distance between lens and image when the object is far away. To
avoid confusion, this focal length is known as the _principal_ focus,
and is denoted by the symbol f. In the middle diagram the object is
quite near the lens, which has to deal with rays striking its nearer
surface at an acuter angle than before (reckoning from the centre). As
the lens can only deflect their path to a fixed degree, they will not,
after passing the lens, come together until they have reached a point,
F^1, further from the lens than F. The nearer we approach O to the
lens, the further away on the other side is the focal point, until a
distance equal to that of F from the lens is reached, when the rays
emerge from the glass in a parallel pencil. The rays now come to a focus
no longer, and there can be no image. If O be brought nearer than the
focal distance, the rays would _diverge_ after passing through the lens.

RELATIVE POSITIONS OF OBJECT AND IMAGE.