Around 1800, the English scientist Thomas Young designed and performed an experiment which produced seemingly unexplainable phenomena. At least, it was unexplainable in terms of the early ``corpuscular'' theory of light. Young observed the image of light passing through first one slit, then two slits closely spaced and parallel to one another. He used filtered light from a mercury arc to insure that he had nearly monochromatic light. The first slit insured that light striking the double slit further on had a definite phase relationship among various points on the wave front (i.e. coherence). The image that Young observed was a series of light and dark areas which did not represent a plain geometrical image of double slits. Furthermore, a point on the screen which was illuminated when one of the double slits was covered became dark when both slits were uncovered. A ``corpuscular'' theory (in which light is comprised of a series of particles) cannot adequately explain this phenomenon. [Not, that is, until the advent of quantum mechanics, in which the distinction between waves and particles became forever blurred.]
We can re-perform Young's experiment and, in fact, eliminate the first slit. If we place the double slits far enough from the light source, the first slit becomes unnecessary. This shows that there is a definite phase relationship between points on the wave front which pass through the double slits. In other words, in a small angle, light from an incandescent source is spatially coherent.