These images — along with one executed by M. Rapine (based on a painting by Alexandre-Blaise Desgoffe), showing the effects of light on a soap bubble — were used to explain the phenomenon of birefringence, or double refraction: the colourful results of light waves moving through material at unequal speeds. And their subjects were not chosen haphazardly. Newton was famously interested in the iridescence of soap bubbles. His observations of their refractive capacities helped him develop the undulatory theory of light. But he was no stranger to feathers either. In the Opticks (1704), he noted with wonder that, “by looking on the Sun through a Feather or black Ribband held close to the Eye, several Rain-bows will appear.”