pitch black, turn out some more lights. Note that the paper appears to be white regardless of the actual amount of light energy that enters the eye.Lightness constancy illustrates an important perceptual principle: Perception is relative. Lightness constancy may occur because the white piece of paper reflects more light than any of the other objects in the room—regardless of the different lighting conditions. That is, you may have determined the lightness or darkness of the paper relative to the other objects in the room. Another explanation, proposed by 19th-century German physiologist Hermann von Helmholtz, is that we unconsciously take the lighting of the room into consideration when judging the lightness of objects. COLOR CONSTANCYColor constancy is closely related to lightness constancy. Color constancy means that we perceive the color of an object as the same despite changes in lighting conditions. You have experienced color constancy if you have ever worn a pair of sunglasses with colored lenses. In spite of the fact that the colored lenses change the color of light reaching your retina, you still perceive white objects as white and red objects as red. The explanations for color constancy parallel those for lightness constancy. One proposed explanation is that because the lenses tint everything with the same color, we unconsciously "subtract" that color from the scene, leaving the original colors. SHAPE CONSTANCYAnother perceptual constancy is shape constancy, which means that you perceive objects as retaining the same shape despite changes in their orientation. To understand shape constancy, hold a book in front of your face so that you are looking directly at the cover. The rectangular nature of the book should be very clear. Now, rotate the book away from you so that the bottom edge of the cover is much closer to you than the top edge. The image of the book on your retina will now be quite different. In ...
