Question: What is a verb for *illusion*? I want to use it in a sentence like the following:

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> The optical effect [illudes] my perception of its real shape.
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> 
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But **illude** does not exist. But I cannot find *illude* in my Dictionary (OS X Dictionary.app). I want to find a verb that is somewhere between *deceive* and *confuse*.

Answer: 
Um, what do you mean “*illude* does not exist”? Sure it does. It’s just a tad rare these days. From the OED:

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> **illude** /ɪˈl(j)uːd/, *v.* Now *rare*. 
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> Also 6 **illud**. 
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> Etymology: ad. L. *illūdĕre* to make sport of, jest or mock at, ridicule, occas. to trick, impose upon, f. *il-* (**il-**1) + *lūdĕre* to play. Cf. obs. Fr. *illuder* (Godef.).
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> **† 1.** *trans.* To mock, make sport of, deride. *Obs*.  
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> **2.** To trick, impose upon, deceive with false hopes.
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There a bit more than that, but those other senses are not used any longer; only #2 is. Here are some citations to go with it:

> 
> * 1670 G. H. *Hist. Cardinals* ɪɪɪ. ɪɪɪ. 293 ― Full of hypocrisie and dissimulation, to lull and illude one another.
> * 1872 M. Collins *Two Plunges for Pearl* I. iii. 64 ― They had allowed their imaginations to illude them.
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They also mention the forms *illuded* and *illuding*; for example:

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> * 1745 Warton *Pleas. Melanch.* 185 ― The woodman’s stroke, or distant tinkling team··alarms The illuded sense.
> * 1887 *Athenæum* 3 Dec. 745/1 ― They [women] come across unfavourable specimens of the illuding sex.
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I’m not saying that you’re apt to skate on entirely thick ice if you were to use these, but *illude* certainly **DOES** “exist”, and means pretty much what you would think it means.
I might use the word *obscures* in that sentence:

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> The optical effect obscures my perception of its real shape.
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From [Oxford Dictionaries](http://oxforddictionaries.com/definition/english/obscure): "obscure (verb): Keep from being seen; conceal".