tude or the angle under which it is viewed is then stated to
be very small. If the object is now moved to the point B, which is only 5
feet from the eye, its apparent magnitude will be found to have increased
to such an extent that we can distinguish not only its shape, but also some
of the marking. When moved to within a few inches from the eye as at C, we
see it under an angle so great that all the detail can be distinctly seen.
By having brought the object nearer the eye, thus rendering all its parts
clearly visible, we have actually magnified it, or made it appear larger,
although its actual size remains exactly the same. When the distance
between the object and the observer is known, the apparent magnitude of the
object varies inversely as the distance from the observer.

Let us suppose that we wish to produce an image of a tree situated at a
distance of 5000 feet. At this distance the light rays from the tree will
be nearly parallel, so that if a lens having a focal length of 5 feet is
fastened in any convenient manner in the wall of a darkened room the image
will be formed 5 feet behind the lens at its principal focus. If a screen
of white cardboard be placed at this point we shall find that a small but
inverted image of the tree will be focussed upon it. As the distance of the
object is 5000 feet, and as the size of the received image is in proportion
to this distance divided by the focal length of the lens, the image will be
as 5000 / 5, or 1000 times smaller than the object.

If now the eye is placed six inches behind the screen and the screen
removed, so that we can view the small image distinctly in the air, we
shall see it with an apparent magnitude as much greater than if the same
small image were equally far off with the tree, as 6 inches is to 5000
{136} feet, that is 10,000 times. Thus we see that although the image
produced on the screen is 1000 times less than the tree from one cause, yet
on account of it being brought near to the eye it is 10,000 times greater
in apparent magnitude; therefore its apparent magnitude is increased as
10,000 / 1000, or 10 times. This means that by means of the lens it has
actually been magnified 10 times. This magnifying power of a lens is always
equal to the focal length divided by the distance at which we see small
objects most distinctly, viz. 6 inches, and in the present instance is 60 /
6, or 10 times.

When the image is received upon a screen the apparatus is calle