Fig. 28
These illusions show how many things that people [{768}] "see with their own eyes" are not so. All depends on the attention and the state of mind at the time when the seeing is done. In day-dreams these illusions often occur and may be the basis of delusions.
Fig. 29
There are, however, a number of optical illusions which illustrate certain defects of our vision that cannot be corrected, no matter how much we may desire to see correctly. We continue to see them not as they are but as they seem, and we must correct our vision by information from other sources. The Müller-Lyer lines are familiar and are given here (Fig. 29) because [{769}] they show how easily the senses may deceive us, even that most acute of our senses, vision, as to the sizes of things.
Fig. 30
Figure 30 illustrates how easy it is to be deceived by the juxtaposition of different portions of objects. I have had a woman who had cut out high collars for children and who happened to put them in the juxtaposition of the sketch here given think that she was either losing her sight or her judgment was being affected by the nervous condition in which she was. Nothing would persuade her that some serious development was not taking place until I showed her this illustration. In this illusion the juxtaposition of the short curved line to the long curved line of the other figure produces all the disturbance of judgment of size.
The illusions of filled and unfilled space are interesting and are quite inevitable. They are due to physiological visual effects and are very strikingly illustrated by what is known as the sun and moon illusion. Both these luminaries seem larger at the horizon than they are at the zenith. This is entirely an optical illusion. The horizon seems farther away than the zenith because vision to it is interrupted. The heavens appear not to be a half sphere, but more like an old-fashioned watch glass.
