392 
Transactions of the Society. 
The expression will be the same in all the investigations, 
and may be called K, in which case the illumination received per 
unit area is 
KS 2 
d 2 
or by an object (0) 
K S 2 O 2 
d 2 ’ 
If S = 10 X 10 mm. = 100 square mm. 
d = 400 mm.^ d 2 = 160,000 mm. 
L 
Then the illumination received by an object from a source of 
light is 
K 1 00 
160,000 
K 
1600 
per unit area 
and this forms a basis for comparison when no bull’s-eye is used. 
Now interpose a bull’s-eye between the source of light and the 
object. 
There are two positions for every lens where an image of a 
source of light can be produced on a screen. 
Tig. 3 (1) shows the bull’s-eye of a focal length of 75 mm. at a 
distance x = 100 mm. from the source (S), and x' = 300 mm. from 
the screen (O). 
