Physiology and Hygiene for Secondary Schools

by Francis M. Walters

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.

Fig. 166—Diagram for proving presence of the blind spot.

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 (Fig. 166). 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[pg 391] 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.

Fig. 167—Model for demonstrating the eyeball.

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.