436 THE OUTGO OF ENERGY 



principal focal distance of any lens in the dioptric 

 system may be found by dividing one metre, or 

 100 cm., by the number of dioptres ; thus the 

 focal distance of a lens of 4 D = ^^ = 25 cm. 



Convex lenses are marked -}-, concave lenses . 



If two or more lenses are placed together, the 

 dioptric power of the system thus formed equals 

 the algebraical sum of the dioptric powers of 

 the lenses in the system. 



