Of all Reflex's, that which is seen on the darkest Ground, must appear the boldest and most sensible; and on the contrary, that appearing on the brightest Ground the dimmest and least distinct: this arises from the Contrast between things of different Obscurities; the least obscure of these serving to set off the others with the greater Lustre, and the brighter to render the others still darker and less perceptible: Just like two things of unequal whiteness, which when opposed to each other, the whitest casts a dimness and foil on the other, beyond what it had of it self.