conditions allow
may be admitted to the eye. Most modern camera lenses are fitted with
adjustable stops which can be made larger or smaller by twisting a ring
on the mount, and are named "iris" stops. The image of anything seen is
thrown on the retina upside down, and the brain reverses the position
again, so that we get a correct impression of things.

THE USE OF SPECTACLES.

[Illustration: FIG. 117_a_.]

[Illustration: FIG. 117_b_.]

[Illustration: FIG. 118_a_.]

[Illustration: FIG. 118_b_.]

The reader will now be able to understand without much trouble the
function of a pair of spectacles. A great many people of all ages suffer
from short-sight. For one reason or another the distance between lens
and retina becomes too great for a person to distinguish distant objects
clearly. The lens, as shown in Fig 117_a_, is too convex--has its
minimum focus too short--and the rays meet and cross before they reach
the retina, causing general confusion of outline. This defect is simply
remedied by placing in front of the eye (Fig. 117_b_) a _concave_ lens,
to disperse the rays somewhat before they enter the eye, so that they
come to a focus on the retina. If a person's sight is thus corrected for
distant objects, he can still see near objects quite plainly, as the
lens will accommodate its convexity for them. The scientific term for
short-sight is _myopia_. Long-sight, or _hypermetropia_, signifies that
the eyeball is too short or the lens too flat. Fig. 118_a_ represents
the normal condition of a long-sighted eye. When looking at a distant
object the eye thickens slightly and brings the focus forward into the
retina. But its thickening power in such an eye is very limited, and
consequently the rays from a near object focus behind the retina. It is
therefore necessary for a long-sighted person to use _convex_ spectacles
for reading the newspaper. As seen in Fig. 118_b_, the spectacle lens
concentrates the rays before they enter the eye, and so does part of the
eye's work for it.

Returning for a moment to the diagram of the eye (Fig. 116), we notice a
black patch on the retina near the optic nerve. This is the "yellow
spot." Vision is most distinct when the image of the object looked at is
formed on this part of the retina. The "blind spot" is that point at
which the optic nerve enters the retina, being so called from the fact
that it is quite insensitive to light. The finding of the blind spot is
an interesting littl