ly and thoroughly to _flood
the eyes with water_. Any of the chemical which remains may then be
counteracted by the proper reagent, care being taken to use a very dilute
solution. To counteract an acid, use sodium bicarbonate (cooking soda),
and for bases use a very dilute solution of acetic acid (vinegar). To
guard against getting the counteractive agent too strong for the inflamed
eye, it should first be tried on an eye that has not been injured.

*Summary.*--The nervous impulses that cause the sensation of sight are
started by light waves falling upon a sensitized nervous surface, called
the retina. By means of refractive agents, forming a part of the eyeball
in front of the retina, light from different objects is focused and made
to form images of the objects upon the surface. In this way the light is
made to stimulate a portion of the retina corresponding to the form of the
object. This, _the image method of stimulation_, enables the mind to
recognize objects and to locate them in their various positions. While the
greater portion of the eyeball is concerned in the focusing of light, the
crystalline lens, operated by the ciliary muscle, serves as the special
instrument of accommodation. Muscles attached to the eyeballs turn them in
different directions, and so adjust them with reference to each other that
double vision is avoided.

*Exercises.*--1. Under what conditions are light waves reflected,
refracted, and absorbed?

2. Why does the body not need a light-producing apparatus, corresponding
to the larynx in the production of sound?

3. How is the light from a candle made to form an image?

4. What different things must happen in order that one may see an object?

5. Make a sectional drawing of the eyeball, locating and naming all the
parts.

6. Of what parts are the outer, middle, and inner coats of the eyeball
made up?

7. What portions of the eyeball reflect light? What absorb light? What
transmit light? What refract light?

8. Show how the iris, the crystalline lens, the retina, the ciliary
muscle, and the cornea aid in seeing.

9. Trace a wave of light from a visible object to the retina.

10. Why does not the inverted image on the retina cause us to see objects
upside down?

11. What change occurs in the shape of the crystalline lens when we look
from distant to near objects? From near to distant objects? Why are these
changes necessary? How are they brought about?

12. How does the method o