in this manner is termed the _conjugate
foci_ of a lens, and though every lens has only one principal focus, yet
its conjugate foci are innumerable.

[Illustration: FIG. 69.]

The formation of an image of some distant object in its principal focus is
one of the most useful properties of a convex lens, and it is this property
that forms the basis of several well-known optical instruments, including
the camera, telescope, microscope, etc.

If we take an oblong wooden box, AA, and substitute a sheet of ground
glass, C, for one end, and drill a small pinhole, H, in the centre of the
other end opposite the {132} glass plate, we shall find that a tolerably
good image of any object placed in front of the box will be formed upon the
glass plate. The light rays from all points of the object, BD, Fig. 70,
will pass straight through the hole H, and illuminate the ground glass
screen at points immediately opposite them, forming a faint inverted image
of the object BD. The purpose of the hole H is to prevent the rays from any
one point of the object from falling upon any other point on the glass
screen than the point immediately opposite to it, therefore the smaller we
make H, the more distinct will be the image obtained. Reducing the size of
H in order to produce a more distinct image has the effect of causing the
image to become very faint, as the smaller the hole in H, the smaller the
number of rays that can pass through from any point of the object. By
enlarging the hole H gradually, the image will become more and more
indistinct until such a size is reached that it disappears altogether.

[Illustration: FIG. 70.]

If in this enlarged hole we place a double convex lens, LL, Fig. 71, whose
focal length suits the length of the box, the image produced will be
brighter and more distinct than that formed by the aperture, H, since the
rays which proceed from any point of the object will be brought by the lens
to a focus on the glass screen, forming a bright {133} distinct image of
the point from which they come. The image owes its increased distinctness
to the fact that the rays from any one point of the object cannot interfere
with the rays from any other point, and its increased brightness to the
great number of rays that are collected by the lens from each point of the
object and focussed in the corresponding point of the image. It will be
evident from a study of Fig. 71 that the image formed by a convex lens must
necessarily