The Eye : Part 6

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 27 of 28)

8. Show how the iris, the crystalline lens, the retina, the ciliate 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 of 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.

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. 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.

To prove the Presence of the Blind Spot. - Close the left eye and with the right gaze steadily at the spot on the left side of this page. Then starting with the book a foot or more from the face, move it slowly toward the eye. A place will be found where the spot on the right entirely disappears. On bringing it nearer, however, it is again seen. As the book is moved forward or backward, the position of the image of this spot changes on the retina. When the spot cannot be seen, it is because the image falls on the blind spot.

Dissection of the Eyeball. - Procure from the butcher two or three eyeballs obtained from cattle. After separating the fat, connective tissue, and muscle, place them in a shallow vessel and cover with water. Insert the blade of a pair of sharp scissors at the junction of the sclerotic rotic coat with the cornea and cut from this point nearly around the entire circumference of the eyeball, passing near the optic nerve. Spread open in the water and identify the different parts from the description in the text. Open the second eyeball in water by cutting away the cornea. Examine the parts in front of the lens.

To illustrate Accommodation. - Paste together the ends of a strip of stiff writing paper (two by five inches) making a ring a little less than three inches in diameter. This is to represent the crystalline lens. Now paste a piece of thin paper (two by seven inches) upon a second strip of the same size, leaving an open place in the middle for the insertion of the paper lens. A flexible piece of cardboard (three by twelve inches) is now bent into the form of a half circle and to its ends are fastened the strips of paper containing the ring. Make a small hole in each of the four corners of the bent cardboard. Through these holes pass two loops of thread, or fine string, in opposite directions, letting the ends hang loose from the cardboard.

When everything is in position, the tension from the cardboard flattens the paper lens, while pulling the strings releases this tension and permits the lens to become more rounded. With this simple device the changes in the curvature of the lens for near and distant vision are easily shown.

D.C. Heath and Co. - Publishers
Original copyright 1909