"It seems to me that the shadows are of supreme importance in perspective, seeing that without them opaque and solid bodies will be indistinct... as to their boundaries..., unless these are seen against a background differing in colour... [E]very opaque body is surrounded and has its surface clothed with shadows and lights... [T]hese shadows are... of varying degrees of darkness... caused by the absence of a variable quantity of luminous rays; and these I call primary shadows... From these primary shadows there issue certain dark rays which... vary in intensity according to the varieties of the primary shadows... I call these shadows derived shadows... [D]erived shadows in striking upon anything create as many different effects as are the different places... And since where the derived shadow strikes, it is always surrounded by the striking of the luminous rays, it leaps back with these in a reflex stream towards its source and meets the primary shadow, and mingles with and becomes changed into it... In addition... many different varieties of the rebound of the reflected rays which will modify the primary shadow by as many different colours as there are different points from whence these luminous reflected rays proceed. ...[V]arious distances ...may exist between the point of striking of each reflected ray and the point from whence it proceeds, and [it acquires] various different shades of colour... in striking against opaque bodies."