be. (2.) If the object approaches within the principal focal distance of the lens, no image will be cast by the lens. To make this plainer we append a diagram (Fig. 110), which shows five positions of an object and the relative positions of the image (in dotted lines). First, we note that the line A B, or A B 1 , denotes the principal focal length of the lens, and A C, or A C 1 , denotes twice the focal length. We will take the positions in order:—
Position I. Object further away than 2f. Inverted image smaller than object, at distance somewhat exceeding f.
Position II. Object at distance = 2f. Inverted image at distance = 2f, and of size equal to that of object.
Position III Object nearer than 2f. Inverted image further away than 2f; larger than the object.
Position IV. Object at distance = f. As rays are parallel after passing the lens no image is cast.
Position V. Object at distance less than f. No real image—that is, one that can be caught on a focussing screen—is now given by the lens, but a magnified, erect, virtual image exists on the same side of the lens as the object.
We shall refer to virtual images at greater length presently. It is hoped that any reader who practises photography will now understand why it is necessary to rack his camera out beyond the ordinary focal distance when taking objects at close quarters. From Fig. 110 he may gather one practically useful hint—namely, that to copy a diagram, etc., full size, both it and the plate must be exactly 2f from the optical centre of the lens. And it follows