What is a self-illustrating phenomenon? A self-illustrating phenomenon is an image that is generated automatically as a result of an experiment. More importantly, it is an image that exposes the phenomenon behind the observation. It often represents an answer to a question.
Here is an example. Ernst Chladni, a musician and amateur scientist, found a way to make visible the vibrations caused by sound waves. He covered metal and wooden plates with sand and ran a violin bow against them. When the plate vibrates, sand collects at the stationary nodes, having been shaken from the moving regions. These nodes are the zero-crossings of the standing wave that produces the sound. Being a musician, he was interested in the theory of sounds and he used the tools at his disposal, in this case a violin bow.
 Chapter 6, Science since Babel: Graphs, Automatic Recording Devices and the Universal Language of Instrument, In Instruments and the Imagination, T. L. Hankins and R. J. Silverman, Princeton University Press, 1995.
 p. 134, The Scientific Image, H. Robin