Again, examining a hat, each hair of which the felt is composed is a small cylinder, which, seen through a microscope, presents a white edge, like that which is seen on a stick of sealing-wax, when viewed in a strong light: this edge, then, remits to the eye white light.

What has just been found to be the case in these two examples is true of every body in nature; it is this white light, reflected from every visible point, which essentially determines the degree of brightness belonging to each portion of the object considered, because these white rays are the most perfect and the most vivid of those remitted to us by each particle; it is these consequently, which enable us best to recognise the form, to appreciate the inclination of each element, and the curvature at each point of the surface.

We are accustomed to this great abundance of white light, and to the services rendered by it to vision; and it is by comparison with it that coloured light is usually judged.