Therefore, although the principal focus of a Lens (as determined by the degree of its convexity) is always the same, yet the focus for objects near at hand varies, being longer as they are brought closer to the Lens.
Formation of a Luminous Image by a Lens.—As the rays of light proceeding from a point are brought to a focus by means of a Lens, so are they when they proceed from an object, and in that case an image of the object is the result.
The above figure illustrates this. The size of the image varies with the distance of the arrow from the glass—being larger and formed at a point further from the Lens as the object is brought nearer. The refracting power of the Lens also influences the result—lenses of short focal length, i. e. more convex, giving a smaller image.
In order that the course pursued by pencils of rays proceeding from an object may be easily traced, the lines from the barb of the arrow in the last figure are dotted. Observe that the object is necessarily inverted, and also that those rays which traverse the central point of the Lens, or the centre of the axis, as it is termed, are not bent away, but pursue a course either coincident with, or parallel to, the original, as in the case of refracting media with parallel surfaces.
SECTION III.
The Photographic Camera.
The Photographic Camera is in its essential nature an extremely simple instrument. It consists merely of a dark chamber, having an aperture in front in which a Lens is inserted. The accompanying figure shows the simplest form of Camera.