e experiment. On a card make a large and a small
spot three inches apart, the one an eighth, the other half an inch in
diameter. Bring the card near the face so that an eye is exactly
opposite to each spot, and close the eye opposite to the smaller. Now
direct the other eye to this spot and you will find, if the card be
moved backwards and forwards, that at a certain distance the large spot,
though many times larger than its fellow, has completely vanished,
because the rays from it enter the open eye obliquely and fall on the
"blind spot."

Chapter XIII.

THE MICROSCOPE, THE TELESCOPE, AND THE MAGIC-LANTERN.

The simple microscope--Use of the simple microscope in the
telescope--The terrestrial telescope--The Galilean telescope--The
prismatic telescope--The reflecting telescope--The parabolic
mirror--The compound microscope--The magic-lantern--The
bioscope--The plane mirror.

In Fig. 119 is represented an eye looking at a vase, three inches high,
situated at A, a foot away. If we were to place another vase, B, six
inches high, at a distance of two feet; or C, nine inches high, at three
feet; or D, a foot high, at four feet, the image on the retina would in
every case be of the same size as that cast by A. We can therefore lay
down the rule that _the apparent size of an object depends on the angle
that it subtends at the eye_.

[Illustration: FIG. 119.]

To see a thing more plainly, we go nearer to it; and if it be very
small, we hold it close to the eye. There is, however, a limit to the
nearness to which it can be brought with advantage. The normal eye is
unable to adapt its focus to an object less than about ten inches away,
termed the "least distance of distinct vision."

THE SIMPLE MICROSCOPE.

[Illustration: FIG. 120.]

A magnifying glass comes in useful when we want to examine an object
very closely. The glass is a lens of short focus, held at a distance
somewhat less than its principal focal length, F (see Fig. 120), from
the object. The rays from the head and tip of the pin which enter the
eye are denoted by continuous lines. As they are deflected by the glass
the eye gets the _impression_ that a much longer pin is situated a
considerable distance behind the real object in the plane in which the
refracted rays would meet if produced backwards (shown by the dotted
lines). The effect of the glass, practically, is to remove it (the
object) to beyond the least distance o