121.

Shadow is the diminution alike of light and of darkness, and stands between darkness and light.

A shadow may be infinitely dark, and also of infinite degrees of absence of darkness.

The beginnings and ends of shadow lie between the light and darkness and may be infinitely diminished and infinitely increased.  Shadow is the means by which bodies display their form.

The forms of bodies could not be understood in detail but for shadow.

122.

OF THE NATURE OF SHADOW.

Shadow partakes of the nature of universal matter.  All such matters are more powerful in their beginning and grow weaker towards the end, I say at the beginning, whatever their form or condition may be and whether visible or invisible.  And it is not from small beginnings that they grow to a great size in time; as it might be a great oak which has a feeble beginning from a small acorn.  Yet I may say that the oak is most powerful at its beginning, that is where it springs from the earth, which is where it is largest (To return:) Darkness, then, is the strongest degree of shadow and light is its least.  Therefore, O Painter, make your shadow darkest close to the object that casts it, and make the end of it fading into light, seeming to have no end.

Of the various kinds of shadows. (123-125).

123.

Darkness is absence of light.  Shadow is diminution of light.  Primitive shadow is that which is inseparable from a body not in the light.  Derived shadow is that which is disengaged from a body in shadow and pervades the air.  A cast transparent shadow is that which is surrounded by an illuminated surface.  A simple shadow is one which receives no light from the luminous body which causes it.  A simple shadow begins within the line which starts from the edge of the luminous body a b.

124.

A simple shadow is one where no light at all interferes with it.

A compound shadow is one which is somewhat illuminated by one or more lights.

125.

WHAT IS THE DIFFERENCE BETWEEN A SHADOW THAT IS INSEPARABLE FROM A
BODY AND A CAST SHADOW?

An inseparable shadow is that which is never absent from the illuminated body.  As, for instance a ball, which so long as it is in the light always has one side in shadow which never leaves it for any movement or change of position in the ball.  A separate shadow may be and may not be produced by the body itself.  Suppose the ball to be one braccia distant from a wall with a light on the opposite side of it; this light will throw upon the wall exactly as broad a shadow as is to be seen on the side of the ball that is turned towards the wall.  That portion of the cast shadow will not be visible when the light is below the ball and the shadow is thrown up towards the sky and finding no obstruction on its way is lost.

126.

HOW THERE ARE 2 KINDS OF LIGHT, ONE SEPARABLE FROM, AND THE OTHER
INSEPARABLE FROM BODIES.