f adjustment, or accommodation, of the eyeball
differ from that of a telescope or a photographer's camera?

13. With two eyes how are we kept from seeing double?

14. What different purposes are served by the tears. Trace them from the
lachrymal glands to the nostrils.

15. Show how the proper lenses remedy short- and long-sightedness.

16. Describe the conjunctiva and give its functions. Why should it be so
sensitive?

17. How may eye strain cause disease in parts of the body remote from the
eyes?

18. How does "image stimulation" differ from light stimulation in general?

PRACTICAL WORK

*To illustrate Simple Properties of Light.*--1. Heat an iron or platinum
wire in a clear gas flame. Observe that when a high temperature is reached
it gives out light or becomes luminous.

2. Cover one hand with a white and the other with a black piece of cloth,
and hold both for a short time in the direct rays of the sun. Note and
account for the difference in temperature which is felt.

3. Stand a book or a block of wood by the side of an empty pan in the
sunlight, so that the end of the shadow falls on the bottom of the pan.
Mark the place where the shadow terminates and fill the pan with water.
Account for the shadow's becoming shorter.

4. Place a coin in the center of an empty pan and let the members of the
class stand where the coin is barely out of sight over the edges of the
pan. Fill the pan with water and account for the coin's coming into view.
Show by a drawing how light, in passing from the water into the air, is so
bent as to enter the eye.

5. With a convex lens, in a darkened room, focus the light from a candle
flame so that it falls on a white screen and forms an image of the candle.
Observe that the image is inverted. In a well-lighted room focus the light
from a window upon a white screen. Show that, as the distance from the
window to the screen is changed, the position of the lens must also be
changed. (Accommodation.)

6. Hold a piece of cardboard, about eight inches square and having a
smooth, round hole an eighth of an inch in diameter in the center, in
front of a lighted candle in a darkened room. Back of the opening place a
muslin or paper screen (Fig. 157). Observe that a dim image is formed.
Account for the fact that it is inverted. Hold a lens between the
cardboard and the screen so that the light passes through it also. The
image should now appear smaller and more distinct.