The Book

Book front cover

The ultimate resource:
A Field Guide To Time-Varying Light Sources

Learn more...

Purchase the book on Amazon!

There are two main techniques for observing time-varying light sources with the naked eye: eyeball jiggling and mirror jiggling. The two videos provided below demonstrate these techniques.


Eyeball jiggling

This video demonstrates how to see time-varying light sources with the naked eye.

See the video transcript...

Eyeball jiggling video transcript

Hello, welcome to this tutorial video from I’m Dan Bennett, and in this video I’m going to teach you about eyeball jiggling. Now, eyeball jiggling sounds like a crazy technique, and it is. And in fact, most people can’t do it. But don’t be discouraged. I’m going to show you what it takes, I’m going to show you some methods; and if at the end of this video you go away and practice and you still find you can’t do it, all is not lost, because there is a second video which is about mirror jiggling, which will allow you to see exactly the same things with the use of a mirror.

Time-varying light sources are lights, such as neon tubes, fluorescent lights, car tail lights, street lights, that kind of thing, which are varying in intensity or color or configuration very, very quickly. But they’re actually varying so quickly that you can’t see the variation with the naked eye when you simply look. You know, you’re in a street at night; you look at the street lights… it does not look like those street lights are flashing on and off 120 times per second. You can’t see that. And the reason is that the image from the light is hitting your retina 120 times per second on the same part of your retina, and so your retina never relaxes and says, “Oh, there’s a moment of darkness.” It’s just permanently hitting your retina all the time, and you retina never sees the dark periods which come 120 times per second. For that kind of light; other lights flash at different rates.

However, the technique of eyeball jiggling will take that image and move it across your retina. So instead of hitting the same part of your retina all the time, it is swept across your retina, leaving a trail. That trail, or trace, is a transient thing – it doesn’t last very long, it maybe lasts a quarter of a second – but you can see it. And if you train yourself to see it, you’ll be able to see it, enjoy it and interpret it, and understand just with a glance what a given time-varying light is actually doing.

Let me show you how to do it. The technique I’m about to show you I call “eyeball jiggling”, because I’m about to jiggle my eyeballs. And here is how it works. What I’m doing at the moment is a very bizarre, very unusual ocular, muscular thing, where I’m moving my eyes up and down regularly very fast. OK, so you’ve probably never seen anyone do that in a video before! It’s a technique that I developed for myself when I was a boy (the book says something about it), and so therefore, I don’t expect you to be able to do it. Some people can. I’ve found some people who can do a motion similar to that, if not identical, and have no trouble doing it. Most of you will really struggle to do anything of this kind.

The objective, though, of eyeball jiggling is simply to move your gaze from one place to another, fairly smoothly, fairly rapidly, without closing your eyelids. And if you can do that, you will start to see time-varying light source traces on your retina. And if you can do it just with your eyes, then you don’t need a mirror, you don’t need a camera; you can see these things wherever you are, whenever you see a time-varying light source in front of you. So, instead of doing a regular oscillation like this, one technique might be to do a single flip, so for example, flick, like that, flick, flick, flick. In fact, I have a technique – basically looks like that: flick, flick, flick, flick. And so I can be driving along at night, and I see a light source, I’ll just go flick, and that will tell me instantly what that light source is doing. Sometimes it’s really interesting; sometimes it’s not. But it’s just a cool way of being able to observe the world without additional equipment.

There are ways of practicing. You can practice by getting someone to move their finger up and down like this, and you start moving your eyes to follow the finger; and then they take the finger away and you see if you can still reproduce that kind of movement. I have to say, most people will probably not be able to, at least to start with. But, it’s something to practice, and once you start developing even the first little pieces of the technique, you’ll be able to start seeing time-varying light sources. Obviously, it’s a good idea to go outside at night, and find some time-varying light sources you can practice on, such as street lights, neon tubes and the like. And that will give you an opportunity to see if those traces really are happening on your retina.

But, like I said, if at the end of this you still can’t do it, never fear. Go ahead and watch my other video on mirror jiggling, and you’ll find another way that you can observe time-varying light sources very satisfactorily at night. Thanks for watching.


end faq


Mirror jiggling

This video demonstrates how to use a mirror to observe time-varying light sources.

See the video transcript...

Mirror jiggling video transcript

Hello. Welcome to this video from I’m Dan Bennett, and in this video I’m going to teach you the technique of mirror jiggling. For those of you who have had a look at my other video on eyeball jiggling, and been dismayed and unable to reproduce that technique, don’t worry: this is the video that’s going to allow you to see time-varying light sources with just very simple equipment, in this case a mirror.

The point of vision jiggling, whether it’s eyeball jiggling or mirror jiggling, is to take the image of a time-varying light source and translate it across your retina. If you imagine this is the retina, at the back of the eye… if I’m looking straight at a time-varying light source then every time that light source flashes it’s going to hit the same part of my retina. Well, with vision jiggling, what you’re doing is you’re making successive flashes move across your retina, which gives you a transient trace which allows you to see, albeit briefly, but very clearly, what that time-varying light source is doing over time. It works really well for things like LED tail lights on cars, LED street lights, fluorescent tubes of certain kinds, neon tubes, CRT televisions, that kind of thing.

If you can’t move your eyes like this, then what you’re going to need is a mirror. This is not my mirror, this is my daughter’s mirror. She is kindly lending it to me for the occasion. And what I’m going to do is I’m going to show you how to use this mirror to cause the image of a time-varying light to move across your retina. In this case, instead of moving your eyes, you’re going to be moving your fingers or your arms.

Let’s say that the video camera is in fact a time-varying light source. It isn’t, but let’s say I’m looking straight at it, and I want to see its behavior using a mirror. How would I do that? The answer is, I put the mirror just below my eye, maybe even rest my thumb on my cheek bone. I tilt the mirror downwards through about 45°, and I look into the mirror at the light source. You should be able to see my eye in the mirror if you look carefully. Then, if I move the mirror like this, I am moving the image of the light source across my retina and I will be able to see that trail developing on my retina, which will give me the trace that I’m looking for that lets me see what the time-varying light source is doing.

So, the technique, remember, is to move the mirror like this, smoothly and rapidly. Oscillate the mirror to move that light source across your retina. It’s very easy. Some people find it difficult to do it with their fingers and might find it easier doing, say, a sideways motion like this by moving their arms. Notice my fingers aren’t doing the work: I’m using my arms. Course, people might think you’re crazy when you’re doing this out on the street, but, hey, it’s a free world. So that’s the essence of mirror jiggling: put the mirror under your eye, look down into it and rock as follows.

Now, not everybody carries a mirror with them all the time, but I’ll bet almost everybody is carrying a cellphone with them. Turns out that a cellphone, while not a perfect mirror, is certainly good enough for the job. So you can put a cellphone under your eye and rock it like that, or like that, and you will see the trace developing on your retina. So, that in a nutshell is how you do mirror jiggling in order to see time-varying light sources. It’s a technique that works very well; and I encourage you to check out the other videos on my web-site,, and also of course to have a look at the book – A Field Guide To Time-Varying Light Sources. Thanks for watching.


end faq

Site design

This site is powered by Joomla! CMS. The template is "Cast" from Joomlage, modified here and there. The gallery is powered by RokGallery. All design by Dan Bennett.


All content on this website, including images, videos, text and material from "A Field Guide To Time-Varying Light Sources" is © 2023 Daniel H. Bennett / / A Bear Peering Round A Rock, and may not be reproduced without permission. All rights reserved to the extent of applicable law. Exceptions: third-party images, which are credited as applicable, third-party embedded videos and public domain images.