ABSTRACT
Figure 3 The Mueller-Lyer illusion Although the line with outward pointing fins looks longer, both lines are really equal in length.
of our eyes are all there but we usually remain unaware of them. This does not, of course, mean that we cannot become aware of these things if we need to, nor that we take no account of themindeed, when looking through the window I must avoid looking at the frame, but so long as it remains unchanging and irrelevant this can be done automatically without difficulty. It is change which causes attention to switch, when the automatic taking into account fails. It is obviously important to survival to respond to changes in the environment, and this is exactly how the nervous system reacts, with a burst of activity in response to changes, settling back to a steady state once monotony prevails. Indeed, when by special techniques the image on the eye is kept in exactly the same place, moving with the eye's movement, it soon disappears altogether, and in very uniform surroundings, such as smooth snow, a form of blindness can develop. Similarly,
Figure 6 Two Necker cubes The upper one can be seen in two different ways with different faces nearest to the observer, and the lower one is more likely to be seen as a regular two-dimensional pattern.
