The shadow cast by an object on a plane will be smaller in
proportion as that object is lighted by feebler rays. Let _d e_ be
the object and _d c_ the plane surface; the number of times that _d
e_ will go into _f g_ gives the proportion of light at _f h_ to _d
c_. The ray of light will be weaker in proportion to its distance
from the hole through which it falls.