Dietrich Zawischa

Intermittence effects, Benham's top

The Benham top has first been marketed as a children's toy in 1894 (C.E. Benham, Nature, 51:113–114, 1894); the underlying effect (now called Prévost-Fechner-Benham effect) has already been known earlier, see
Fechner, G.T. (1838) Über eine Scheibe zur Erzeugung subjectiver Farben. In: Poggendorfs Annalen der Physik und Chemie pp 227–232. Verlag von Johann Ambrosius Barth, Leipzig

Benham top: Double-click will start the animation, a single click switches it off again

Not all browsers and image-viewing programs show the animation with the correct speed. In this animation, the top moves in 60º jerks, each step should last 0.02 to 0.03 seconds. Mozilla Firefox and also Opera 9.6 work very well on my computer; Internet Explorer and Netscape Navigator are much too slow (in 2010). When opening the image file with ACDSee it performs nicely, with IrfanView too slow. The change of frames must be so fast that the separate phases are not perceived separately, however, there must be a noticeable flicker.
Of course there is the possibility to print the top's pattern, fix it to a real top and then perform the experiment.
If it works and you stare at the animated image for a while, it may happen that in the left picture you see the flickering outer rings reddish-brown, the inner ones dark blue and those in the middle slightly green, and in the right example in the opposite order. There are many factors which influence the appearance of the top, e.g. the general illumination level of the room, and also the size of the disc, and therefore you find below a magnified specimen. I see the effect better on the large disc, but, as already said, with Mozilla or ACDsee (version 3.1).
Caution! Flickering illumination may cause epileptic fits in predisposed people. May be flickering images too? If you are sensitive to that, please do not stare at the animated picture!

benham top 3

Benham top: a double click starts the animation, a single click switches it off.

If the demonstrations shown here are rendered too slowly and you do not want to install an other browser: there are some nice Java applets and shockwave demonstrations in the Web. (If you are interested in this kind of effects, you will like the pages of Michael Bach and of Akiyoshi Kitaoka.)