[Fig. 166]

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 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]

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.

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.

CHAPTER XXIII - THE GENE