Seemingly deceitful

“The Devil’s Trident” et al.

Simply put an illusion is something that is not real. But we most of us want to believe in things that science and reason tell us, by verifiably replicable experiments and observations, are not real; are “illusionary.” They say we see with our eyes, but let us be clear here, our eyes simply act as conduits to our brains, it is our brains that decipher and decide. For those fortunate enough not to be blind, we do like to be titillated by objects of beauty, panoramic views, the flickering of a fire, the waves rolling in and, optical illusions. The latter are a subject of much interest, writing in 1976, Coren et al. (1976, p. 129) pointed out that in the 120 years since Johann Joseph Oppel published the first systematic work on visual geometric illusions, “nearly a thousand papers have appeared that deal with distortions evoked by simple two-dimensional patterns of lines.” In the past 20 years with the aid of computers (to analyse) and social media (to share) the interest in optical illusions has grown further still (Alter, 2013; Hogenboom, 2015; Schultz, 2013). In this short essay I will discuss a number of such illusions, the way/s in which they trick the brain (alongside the human visual system) and the psychology behind them. But first I will discuss the workings of the eye and how it, like the human brain, is very susceptible to chicanery and trickery—to being deceived.

In the beholder’s eye

We know that the human eye works like a camera. When we look at something, light reflected from that thing enters the eyes through the pupil. Bizarrely it comes in upside down and this light and colour info is focused through the optical bits and bobs within the eye (see Appendix A). As Whitaker et al. (1996, p. 2957) point out in relation to the typical functioning of the human eye:

Judgment of the relative position of objects is an important feature of the human visual system. We seem able to perform this task effortlessly across spatial scales. Thus, whilst we can view two objects and estimate their separation, we are also aware of the relative position of internal features of the objects themselves.

However, the human eye, as with the human mind can easily be tricked. Optical illusions occur because our brains automatically try to interpret and make sense of what we see, usually they get things right, that Apple iPad ‘is’ an Apple iPad but, magicians and sellers of snake oil (and psychologists and visual artists) have long known eyes can be tricked because the human brain is partial to seeing myth as fact and fact as fake. Optical illusions fool our brains into seeing things which are there when they are not actually there or are not seemingly there when, in fact, they actually are.

Perception—what we think, what we think we see—is the interpretation of the things that enter our minds including via our eyes. They say beauty is in the eye of the beholder. Beauty is subjective. Therefore, the same object will not be seen as the same thing by any two people. As pointed out by Hogenboom (2015) Aristotle wrote that, “our senses can be trusted but they can be easily fooled.” This was in the context of him looking (not into Nietzsche’s abyss, but) into a waterfall for too long. He observed that, if we watch something moving quickly for too long, and then look at an inanimate object next to it — like the rocks beside a waterfall — they will appear to move in the opposite direction; a phenomena now classified as the “motion aftereffect” or, “the waterfall illusion.” Neuroscientists have argued that this can be explained by the fact that it takes a lot of energy and effort for the eye to compute fast moving and continually ‘forward’ moving objects so that when it suddenly switches to seeing a stationary object if over emphasises the lack of movement and moves the object slowly ‘backward’ (Hogenboom, 2015).

There is a theory attached to all of this and it is called the Centroid Hypothesis. It states that judgments of distance between visual objects are influenced by the brain’s computation of the “centroids of the luminance profiles of the objects” (Whitaker et al., 1996). Concerning the Devil’s Trident (see Appendix B), the Müller-Lyer arrows (see Appendix C), the Penrose triangle (see Appendix D) and similar illusions, the pattern of neural excitation evoked by contextual flank overlaps with that caused by the stimulus terminator, thereby leading (due to the shift of the centroid of summed excitation) to its perceptual displacement. The relative displacement of all stimulus terminators leads to misjudgement of distances between them; that is, the illusion occurs as a side effect due to necessarily low spatial resolution of the neural mechanism of assessment of the relative location of the visual objects.

The Devil’s Trident 🔱

Devil's Trident -- an optical illusion
Figure 1: “The Devil’s Trident,” Masterton and Kennedy (1975, p.107).

The Devil’s Trident (a.k.a., “The Impossible Trident”) was first noted in the academic press by an American psychologist—Donald Schster—who is said to have been inspired by an advert he saw in a magazine (Schuster, 1964). Accounts of the “Devil’s Trident”—see Appendix B—stress that the middle prong, “appears to be in two places at the same time” and that it involves, “incompatible surface depth cues linked as though they were compatible” (Masterton & Kennedy, 1975, p.107).

Knowledge of optical illusions is not a recent thing. Like all good things, we can go back to Ancient Greece to find initial thinking on the subject: Aristotle and the waterfall (as mentioned above). Indeed, as Bach and Poloschek (2006 p. 21) say, Plato also alerted us to the discrepancy between perception and reality in his “Allegory of the Cave.” Philosophers remain intrigued to this day. As Donaldson (2017) argues that impossible figures prove problematic for sense-data accounts of perception that contend that, corresponding to every visual human experience, there are mental objects (sense-data) that we are aware of—and that sense-data have the properties that the objects that our experiences tell us they do. The problem is that sense-data would have to be impossible objects … surely, impossible objects can’t exist!” There are other explanations. For example, illustrations like the Müller-Lyer arrows (see Appendix C) confuses the brain (in some cultures, according to Alter (2013), not all) and it overcompensates, “making the line appear bigger — as it would have to be in real life to produce those kinds of proportions” (Hogenboom, 2015).

Concluding remarks

To sum up, we can assume that we will never be able to suddenly see true reality with crystal clear clarity and 20/20 vision, be it the waterfall’s rock, the shadow play in the cave or the Devil’s Trident. But this is the magic of reality. This is something we should embrace and revere, not fear. Just because we do not know what lies within black holes or what exists beyond the edge of the universe does not mean we need to create myths to fill in the gaps and then dogmatically and religiously follow them (see Dawkins, 2011). Personally, I like that art can trick us it actually says to me we are human. Also, I feel that visual illusions are actually logical and explainable by reason and science: our brains have evolved to need to constantly predict what is about to happen so, illusions demonstrate our brain being logical and telling us what we should ‘typically’ see not what we rarely physically see.


Alter, A. (2013). Are These Lines the Same Height? Popular Science. Retrieved,

Bach, M &, Poloschek, C. (2006). Optical Illusions. Visual Neuroscience, 6(2), 20-21.

Coren, S., Girgus, J., Erlichman, H., &, Hakstian, A. (1976). An empirical taxonomy of visual illusions. Perception & psychophysics, 20(2), 129–137.

Dawkins, R. (2011). The Magic of Reality: How We Know What’s Really True. London: Bantam Press.

Donaldson, J. (2017). “Impossible Trident” in F. Macpherson (ed.), The Illusions Index. Retrieved,

Hogenboom, M. (2015). How your eyes trick your mind. BBC. Retrieved,

Howey, T. (2016). “How the eye works” Retrieved,

Masterton, B. &, Kennedy, J. (1975). Building the Devil’s Tuning Fork. Perception, 4(1), 107–109.

Schuster, D. H. (1964). A new ambiguous figure: A threestick clevis. The American Journal of Psychology, 77(4), 673.

Schultz, C. (2013). Are Optical Illusions Cultural? Smithsonian. Retrieved,

Whitaker, D., McGraw, P. V., Pacey, I., & Barrett, B. T. (1996). Centroid analysis predicts visual localization of first-and second-order stimuli. Vision Research, 36(18), 2957–2970.

Wikipedia (2020a). “Impossible trident.” Retrieved,

Wikipedia (2020b). “Müller-Lyer illusion.” Retrieved,

Wikipedia (2020c). “Penrose Triangle”. Retrieved,

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Appendix A

How the Eye Works
(Howey, 2016)

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Appendix B

Devil's Trident -- an optical illusion
Note: The devil’s trident (or ‘tuning fork’) is a drawing of an impossible to physically construct object. As articulated by Wikipedia (2020a), “it appears to have three cylindrical prongs at one end which then mysteriously transform into two rectangular prongs at the other end” (see also: Masterton & Kennedy, 1975, p.107).

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Appendix C

Note: The Müller-Lyer illusion typically comprises of three arrows and we think the stems of these arrows are different lengths but, as the diagram shows, they are in fact the same length; as far as we know it was first devised by the German sociologist Franz Müller-Lyer in the late 1800s (Wikipedia, 2020b).

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Appendix D

Impossibility in its purest form” is how the so-called Penrose Triangle is described. It is another of these object which can be drawn but cannot exist as a solid object -- just like this paper’s 'Devil’s Trident'
Note  “Impossibility in its purest form” is how the so-called Penrose Triangle is described. It is another of these object which can be drawn but cannot exist as a solid object (Wikipedia, 2020c) — just like this paper’s “Devil’s Trident” (see Appendix B, above).