III. Six Books on Light and Shade: 135.
135.
OF LIGHT AND LUSTRE.
What is the difference between light and the lustre which is seen on the polished surface of opaque bodies?
The lights which are produced from the polished surface of opaque bodies will be stationary on stationary objects even if the eye on which they strike moves. But reflected lights will, on those same objects, appear in as many different places on the surface as different positions are taken by the eye.
WHAT BODIES HAVE LIGHT UPON THEM WITHOUT LUSTRE?
Opaque bodies which have a hard and rough surface never display any lustre in any portion of the side on which the light falls.
WHAT BODIES WILL DISPLAY LUSTRE BUT NOT LOOK ILLUMINATED?
Those bodies which are opaque and hard with a hard surface reflect light [lustre] from every spot on the illuminated side which is in a position to receive light at the same angle of incidence as they occupy with regard to the eye; but, as the surface mirrors all the surrounding objects, the illuminated [body] is not recognisable in these portions of the illuminated body.