The Photometry of a Bull’s-Eye Lens. 
391 
concentrated upon the object. In this case daylight having only 
an intrinsic brightness of 2 to 4 candle-power per square inch is 
seldom sufficiently intense to give the brilliancy that is required. 
The case will be considered, therefore, of a small brilliant 
artificial source of light. 
Supposing a source of light (s) has an area of S 2 . Suppose 
each unit of light has a brightness of (i). A point of light with a 
brightness of ( i ) will spread out the whole of its light over an 
imaginary sphere with a radius of curvature equal to the distance 
(cl) from the light point to the object. The area of this sphere is 
4 7 r <P, and the light received per unit area of the sphere from each 
point of a source of light is 
i 
4 t r d 2 
or from the total area of the source of light 
