. <--------- Separation, S --------> + . +Now, look at the dot above with your RIGHT EYE (close your left eye), and adjust the distance between your eye and the computer screen. Keep looking at the dot, and you will notice that the + will dissappear. When you are VERY close to the screen, the plus appears; as you back up, it disappears, and as you continue to back up, it reappears.

Your job is to measure the distance from your face to the screen at the distance for which the plus disappears. This distance is D, and it is the DV (Dependent variable) of this study.

Your experiment should have at least three levels of the IV: separation between dot and plus: S = 3 inches, 5 inches, and 7 inches.

1. Plot the distance at which the plus disappears (D, on ordinate) against the separation between the dot and plus (S, on abscissa). Your assignment is to explain (give a theory) why is this plot a LINEAR function?

2. In order to make the dot disappear, you can look at the plus with your LEFT EYE, and vary the distance to the screen. Why do you have to switch your focal point when you switch eyes? Give an explanation that shows WHY you can make the plus disappear with your RIGHT eye and why you can get the dot to disappear in the LEFT eye.

3. Can something fall in the blind spot in BOTH EYES? Show why or why not.

4. How would you find a "new" blind spot (e.g., the result of disease or accident)?

Emmerts Law is written: S = RD, where S = perceived (psychological) size, R = retinal size of the region that is bleached in the retina; D = perceived (psychological) distance from the perceiver (the person who has the afterimage) and the screen.

To do this experiment, look at a piece of cardboard (or other material) with a 1/4 inch hole in it from a distance of 1 foot. Look with the RIGHT EYE ONLY and CLOSE YOUR LEFT EYE. Turn on a 60 Watt bulb for 1 second and turn it off. Blink and you should see an afterimage.

Look at the wall from a distance of 2 feet, 4 feet, 8 feet, or 16 feet and judge the size of the afterimage projected on the wall. Blink and open both eyes, judge how "big" is the afterimage as if it were something real on the wall.

The dependent variable is your judgments of SIZE. You are to estimate the size in inches. If needed, look at a ruler first.

Give yourself another afterimage (after the first one has faded) in your LEFT eye. Use a hole in the cardboard of size 1/2 inch (2/4 inch), also from a distance of 1 foot. Turn on the light and off. Be sure to keep your RIGHT EYE CLOSED.

Repeat the estimations of size from the same distances. Your data (Size Estimates) will fill in the table below. In each box, write down your estimate of how big (size in inches) is the diameter of the afterimage if it were on the wall.

Distance to WALL | ||||

SIZE of Hole | 2 feet | 4 feet | 8 feet | 16 feet |

1/4 inch | ||||

2/4 inch |

1. Now draw a graph with distance on the abscissa (x-axis) and plot your judged sizes as a function of distance, with a separate curve for each size of hole. Each row of the table produces one curve.

Describe the shape of the curves. Does S increase with D? Does S increase with R?

2. Do the data agree with the theory (Emmert's Law)?