of a Plane due to a Small Source of Light. 465 



For the illumination at G = DL -^MGL Multiply both 

 sides by (LE) 2 , ® L ^ J 



2 _- 



.'. (LE) 2 (illumination at G) = DL . (tj^Ycos MGL 



= DLcos 3 MGL 



= MK by the construction. 



Fig. 2. 



Fig. 3. 



Hence the illumination at G due to the source L is pro- 

 portional to the length intercepted between the horizontal 

 line ML, and the curve passing through the points obtained at 

 K is obtained. 



