Owing to historically saved up experience, on the one hand, and thanks to an arrangement of the line connecting eyes of the person in the horizontal plane with another, the person possesses ability more precisely to measure by eye horizontal distances, than height of subjects. Therefore most of people possesses ability to exaggerate vertical extents in comparison with horizontal, and it also leads to sight illusions. If to suggest a number of persons to draw vertical and horizontal lines of identical length, in most cases nacherchenny vertical lines will be shorter than the horizontal. At division approximately of the vertical line in half usually the middle appears too highly (it clearly is visible from brought.
The optical focuses and mysterious ghosts created by means of mirrors, projectors and other technical devices, and also the interesting optical phenomena sometimes observed in the nature (mirages, the polar lights) do not belong to illusions of sight. Emergence of the last is caused by optical properties of the terrestrial atmosphere. Our eye is mistaken in all these cases because it deliberately deceive or by means of technical adaptations, or at the expense of a special condition of the Environment between an eye and object of supervision. Also are not illusions of perception the optical illusions arising at some people in twilight and darkness when insufficient lighting complicates work of eyes and creates special mood and distortions of feelings as a result of short-sightedness, far-sightednesses, daltonism and other defects of the visual device, not characteristic for most of people.
The perception is a reflection of subjects and phenomena in total of their properties and parts at their direct impact on sense organs. It includes last experience of the person in the form of representations and knowledge.
Speaking generally about the reasons of visual illusions (mistakes, deception, it is necessary to specify, first that sometimes they appear owing to specially created, special conditions of supervision, for example: supervision by one eye, supervision at motionless axes of eyes, supervision through a crack, etc. Such illusions disappear at elimination of unusual conditions of supervision.
At the movement of object of supervision also a number of visual illusions which are caused by some properties of our visual device meets. For example, if the circle with the painted sector to bring into rotation, all circle seems to us painted. It can be explained with ability of our eye during fractions of a second to keep visual impression though the visible subject already disappeared from a look.
Secondly, the vast majority of illusions of sight arises not because of optical perfection of an eye, and because of false judgment about visible therefore it is possible to consider that deception arises here at comprehension of a vision. Such illusions disappear at change of conditions of supervision, when performing the elementary comparative measurements, at an exception of some factors disturbing to the correct perception.
At the time of antiquity of people threw into confusion that fact that on the horizon the moon and the sun seem more than when they are highly in the sky. This optical illusion received the name of illusion of the moon. All effect consists that existence of the earth makes impression that the moon at the horizon is further, than the moon in a zenith as the filled space between the observer and the horizon makes impression of bigger extent, than unoccupied space between the observer and the sky over the head. Therefore it seems to us that the moon on the horizon looks more, than the ascended moon.
It is known that imperfectly and sometimes we see our sight not that exists actually. But that fact that huge most of people receive sometimes identical wrong visual impressions, says about objectivity of our sight and that it supplemented by thinking and practice supplies to us with rather exact information on subjects of the outside world. On the other hand, that fact that different people in the course of visual perception possess various ability to be mistaken, sometimes see in subjects what others do not notice, speaks about subjectivity of our visual feelings and about their relativity.