HOW TO KNOW WHICH SIDE OF AN OBJECT IS TO BE MORE OR LESS LUMINOUS
THAN THE OTHER.

Let _f_ be the light, the head will be the object illuminated by it
and that side of the head on which the rays fall most directly will
be the most highly lighted, and those parts on which the rays fall
most aslant will be less lighted. The light falls as a blow might,
since a blow which falls perpendicularly falls with the greatest
force, and when it falls obliquely it is less forcible than the
former in proportion to the width of the angle. _Exempli gratia_ if
you throw a ball at a wall of which the extremities are equally far
from you the blow will fall straight, and if you throw the ball at
the wall when standing at one end of it the ball will hit it
obliquely and the blow will not tell.

[Footnote: See Pl. XXXI. No. 4; the sketch on the right hand side.]