The Zöllner Illusion was created by Johann Karl Friedrich Zöllner (1834 - 1882), a German astrophysicist with a keen interest in optical illusions. Zöllner was inspired by a cloth pattern that he observed in his father’s factory, and first published the illusion in the journal Annalen der Physik in 1860.
The Zöllner Illusion is one among a number of illusions where a central aspect of a simple line image – e.g. the length, straightness, or parallelism of lines – appears distorted in virtue of other aspects of the image – e.g. other background/foreground lines, or other intersecting shapes. These are sometimes called ‘geometrical-optical illusions’. You can search for other geometrical illusions in the Illusions Index
There are a number of general hypotheses about human vision which would explain the Zöllner Illusion. One is that our perceptual systems have a tendency to ‘expand’ acute angles—that is to represent them as larger angles than they really are. In other words, the shorter lines which laterally intersect the longer lines cause the visual system to enhance the orientation contrast between the long and shorter lines (hence ‘expanding’ the acute angles), in turn causing one end of the longer lines to seem closer than the other, hence making the lines appear unparallel (Kitaoka & Ishihari 2000; Eagleman 2001).
One piece of evidence which might support that acute-angle expansion hypothesis is that a variant of the Zöllner illusion in which the long lines are replaced with square dots plotted along the same path, which removes the illusory effect as demonstrated below (this image is taken from Westheimer 2008):
The Zöllner Illusion is one of many illusions which provide data to help illuminate the working of our perceptual systems. Moreover, philosophers will cite illusions of this type in debates about whether we are directly acquainted with the world when we seem to be experiencing it, or whether we are only directly acquainted with are mental representations. In the Zöllner Illusion, for example, if we are aware of non-parallel lines, these are not the long lines in the image, for those are parallel.
The Zöllner Illusion is also interesting because it is relevant to debates about modularity, cognitive penetration, and the nature of experience. To explain: on the hypothesis that the mind is modular, a mental module is a kind of semi-independent department of the mind which deals with particular types of inputs, and gives particular types of outputs, and whose inner workings are not accessible to the conscious awareness of the person – all one can get access to are the relevant outputs. So, in the case of the Zöllner Illusion, a standard way of explaining why experience of the illusion persists even though one knows that one is experiencing an illusion is that the module, or modules, which constitute the visual system are ‘cognitively impenetrable’ to some degree – i.e. their inner workings and outputs cannot be influenced by conscious awareness. For a general discussion of cognitive penetration, see Macpherson (2012).
Philosophers have also been interested in what illusions like the Zöllner Illusion can tell us about the nature of experience. For example, in the case of experiencing the Zöllner Illusion, it would seem to be that the one can know that the lines are parallel whilst at the same time one experiences them as unparallel. If so, then this might count against the claim the perceptual states are belief-like, because if perceptual states were belief like then, when experiencing the Zöllner one would simultaneously believe that the lines were, and were not, parallel. This would seem to entail that one was being irrational, because one would simultaneously be holding contradictory beliefs. But it seems highly implausible that one is being irrational just in virtue of under going this illusion. For discussion of this general point about whether perceptions are like beliefs, see Crane & French (2016).