Newton tracked a narrow beam of white light from a small source going out through a prism and being split up into the different colours. If the reverse of this is imagined, when an observer is looking at something through a prism, a single ray of white light coming to the eye is assembled from rays of different colours, which have actually come from different starting points on the object being viewed. Goethe registered this from the beginning and it drew his attention to the process of perception as a process of synthesis.
This effect only becomes noticeable when there is an edge.