A Visual Illusion Is Not A Hallucination
A visual illusion from most common understandings is seeing something that is not really what is there. Our eyes perceive it as being there and other people would also agree but on closer examination, what we saw was not actually the total reality of the object. It is also interesting to note that visual illusions have nothing really to do with psychological tests used to stimulate analysis or evoke memory from a person or to assess intellect. Actually, a visual illusion is the real presentation of an object that can be seen and explained by anyone in similar language and they can all be wrong in their assessment of the object or situation. The fact that more than one person can see the same visual illusion differentiates it from an hallucination which is strictly personal or seen only by you.
For example, if you were laying out in your backyard that had a privacy hedge and you observed your neighbors head bouncing up and down, you might see his head and assume he was trying to spy on you. If you actually got up and went over to his side of the fence, you might see your neighbor on a pogo stick bouncing up and down. Your neighbor was not trying to look at you surreptitiously, he was simply enjoying his workout on a pogo stick. That might be an oversimplification of a visual illusion but more complicated processes would need more scientific explanation of how what we see affects our perception of reality and consequently our judgement. If you went to your doctor’s office and he held a finger straight up at you between your eyes and asked you to tell him when you saw two fingers instead of one, you would be seeing a visual illusion. If he held two fingers up and asked when you saw one that would also be a visual illusion. The complicating factor is that the doctor is assessing your peripheral vision and whether you might be having eye problems. A really fun visual illusion is to see a giant oblong balloon at night and to call out a few neighbors. The balloon might look like a strange flying machine but if no one tells you that it is a balloon, you might start thinking that it was a flying saucer.