"In discussing the role of light in human vision in the opening section of the treatise, Ghiberti makes a distinction among three kinds of bodies according to how they relate to light.

The first are bodies that radiate light (corpi luminosi), such as the sun, fire, or some precious stones.
The second are bodies that are opaque (corpi umbrosi), do not accept light, and come mostly from the earth as solid and dark matter.
The third, situatied as it were in between, are diaphanous bodies (corpi diafani), such as air, water, glass, crystal, chalcedon, and beryl, through which light can penetrate."