<p>I stumbled upon this phenomenon in the 90s, when I was still a kid. I noticed that in the evening under indirect artificial lighting the red letters on a blue book cover (I think it was a Pink Panther book) seemed to wobble and dance around when you shook the book. At the time, none of the adults knew what it was called and many didn't believe me and refused to look at the book.</p>
<p>Because of related interests I've revisited the issue and managed to recreate the phenomenon on my laptop. The effect is considerably more convincing when it's done in the evening with the screen brightness turned down. As it turns out, the colour of the letters was irrelevant. To get an example of this effect, take any text, set the page colour to black and the text colour to some barely perceptible grey and put it on full screen (F11 in most applications). Now wait and give your eyes a moment, just until you can see the text clearly, and then shake the screen. The mouse cursor will appear to move in comparison to the text. For me it also works if I select one or two letters, in which case shaking the screen will appear to make the selected letters dance around with respect to the rest of the text.</p>
<p>I'm convinced at this point that it's related to the Pulfrich effect in mechanism, but in this case no darkened glass is used and instead of depth motion, motion in the plane is created. Considering how easy it is to find, I assume it's been described before and I'd like to read about it, but I don't know what it's called – hence my question.</p>
<p>Edit: I made a demonstration image; to be viewed in a dark environment with the monitor turned down so the background is just about visible.</p>
<p><a href="https://i.sstatic.net/MGKzN.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/MGKzN.png" alt="Demonstration image" /></a></p>

CC BY-SA 4.0
2021-04-08T11:07:25.270Z
Welcome. Could you visually include the illusion in the question by inserting a screenshot, or the likes?
AliceD
CC BY-SA 4.0
2021-04-08T23:32:03.370Z
Sure. How do I upload on this site?
Anonymous
CC BY-SA 4.0
2021-04-09T09:48:07.433Z
See https://stackoverflow.blog/2010/08/18/new-image-upload-support/ for information on adding images to your question.  In the [potentially insecure mobile app](https://meta.stackexchange.com/a/356369/349643), there is an icon of a camera to do the same,
Chris Rogers
<vision><optical-illusion>
When one wiggles a dark object with bright letters &c on it, the letters appear to move, floating in the plane. What is this optical illusion called?
Anonymous
# When one wiggles a dark object with bright letters &c on it, the letters appear to move, floating in the plane. What is this optical illusion called?

(Asked by: username_0 on 2021-04-08)

I stumbled upon this phenomenon in the 90s, when I was still a kid. I noticed that in the evening under indirect artificial lighting the red letters on a blue book cover (I think it was a Pink Panther book) seemed to wobble and dance around when you shook the book. At the time, none of the adults knew what it was called and many didn't believe me and refused to look at the book.

Because of related interests I've revisited the issue and managed to recreate the phenomenon on my laptop. The effect is considerably more convincing when it's done in the evening with the screen brightness turned down. As it turns out, the colour of the letters was irrelevant. To get an example of this effect, take any text, set the page colour to black and the text colour to some barely perceptible grey and put it on full screen (F11 in most applications). Now wait and give your eyes a moment, just until you can see the text clearly, and then shake the screen. The mouse cursor will appear to move in comparison to the text. For me it also works if I select one or two letters, in which case shaking the screen will appear to make the selected letters dance around with respect to the rest of the text.

I'm convinced at this point that it's related to the Pulfrich effect in mechanism, but in this case no darkened glass is used and instead of depth motion, motion in the plane is created. Considering how easy it is to find, I assume it's been described before and I'd like to read about it, but I don't know what it's called – hence my question.

Edit: I made a demonstration image; to be viewed in a dark environment with the monitor turned down so the background is just about visible.

[![Demonstration image](https://i.sstatic.net/MGKzN.png)](https://i.sstatic.net/MGKzN.png)

**Tags:** vision, optical-illusion

> **Comments:**
> **username_1**: Welcome. Could you visually include the illusion in the question by inserting a screenshot, or the likes?  
> **username_0**: Sure. How do I upload on this site?  
> **username_2**: See https://stackoverflow.blog/2010/08/18/new-image-upload-support/ for information on adding images to your question.  In the [potentially insecure mobile app](https://meta.stackexchange.com/a/356369/349643), there is an icon of a camera to do the same,  

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## 0 answers

