Afterimage

Tricky Atari optical illusion

Afterimage is a short listing that tests a simple but interesting optical illusion. Most users will apparently still ''see" an image after it has been removed. This BASIC program works on all 8-bit Atari computers of any memory size, with disk or cassette.

One day my biology teacher showed the class how an image--such as the bright glare of a flashbulb---can be temporarily "burned" into the retina of the eye. So that after the image has been removed, you canapparently still "see" it. I decided to try demonstrating this effect with the graphics of my 8-bit Atari computer.

Type in Listing 1, AFTIMAGE.BAS, check it with TYPO II and SAVE a copy before you RUN it. When you RUN the program, you'll see a brief title screen and be asked if you need instructions. Press either [Y] or [N].

Now you'll be prompted to choose a color and a shape. The screen then turns white and the shape appears at the center. Concentrate on it and try not to blink. In about 15 seconds the shape will leave the screen. If the test works, you'll still see it--but in the opposite color. So if you picked a red square, you should now see a green square in its place. After several seconds, the program returns to the color selection menu.

Nick Koziupa is a high school sophomore in Hamilton Township, New Jersey. He has been Programming on his Atari for three years.

Listing: AFTIMAGE.BAS Download