First-Hand:Illusion or Reality? Optics and the Human Brain, an Experiment


Submitted by J. Coleman White

When I was studying at Cornell University, many years ago, I had a favorite professor Everett M. Strong—who specialized in lighting. One afternoon, as I headed home from an electrical engineering laboratory, I ran into him in the hallway. After a short chat, he asked me if I had a few minutes. He had something he wanted to show me.

I had the time, so he led me to his laboratory where he had the following relatively simple set-up. There was a conventional movie screen. About eight feet away from it, there was a battery of light projectors, consisting of a central projector surrounded by six others. In between was a black circular piece, probably cardboard, supported on a thin metal rod. When any projector was on, the screen would be cast in light, except for a circular shadow caused by the intervening piece.

He began by turning on the central projector, which produced essentially white light. The screen was then quite white, and the spot quite black. Then, he turned on a second projector, which had a very faint gelatin of red color over the lens. The amount of light coming from this second projector was adjusted to be quite low. What happened when this second projector was turned on was that the screen didn't appear to change at all. It still looked completely white. However, the change in the spot was truly remarkable! It had turned a bright green color. When I gasped at this, I was handed a cardboard tube, and I was told to look at the spot through it. I did so, and found that it was quite black! The apparent green color of the spot was only in my mind. It recognized the slight reddish tint of the background and inserted the complementary color on the spot.

Each projector had a gelatin of one of the primary or secondary colors, and the process was repeated for each. In each case, the spot appeared to be brilliantly colored with the complementary color being projected.

Then, as sort of a finale, Professor Strong turned on all seven lights. What resulted was, again, a seemingly perfectly white screen, but this time a spot that looked, more than anything else, like the rose window in a cathedral. It was covered with arc shaped segments, each of a different brilliant color. The delineation between the segments was quite sharp. However, a look through the cardboard tube confirmed that, in reality, the spot on the screen was totally black.

This, of course, led to an informal lecture on the meaning of what I had seen. Several points were made:

  • When we look at a tree, we see the green leaves, but we also, through our own minds, put red (slightly) in all the shadows cast by each leaf. The child not knowing this uses only green, so the tree looks unnatural. The professional artist knows that it is necessary to put in the red, although he/she may not know why.
  • This is why the worst color of golf tee to buy is dark red. You will stand a much better chance of losing it than you would a yellow or white one, for example.
  • Another common case is shadows on snow. This is easy to test out. The sunlight is slightly amber in color. This causes us to color the shadows on the snow a definite blue even though a look through a tube, to block out the sunlight from view, would confirm that the shadows are really dark gray or black in color, and not blue at all. The moral of all this: When we look at something, how much of the scene is physically there, and how much have we modified?