ELEMENTARY PHOTO -MICROGRAPHY. 137 



of negative from lens. Thus if a negative be 

 enlarged twice, the distance of image from lens 

 will be twice that of the negative from lens. 



D. Distance of negative from lens. 



P. Distance of image from lens. 



N. Number of times negative is to be enlarged. 



F. Focus of lens. 



P = (N + i) F 



CONJUGATE Foci. 



When one of these distances is known, it is easy 

 to find the other, which is called the conjugate focus. 

 ONE OF THE CONJUGATE Foci BEING GIVEN, TO 



FIND THE OTHER. 

 F. Focus of lens. 



D. Distance from lens when object is on the 

 known side. 



D x F 



^ _ P = conjugate focus. 



Example. What should be the distance of 

 negative from lens of 6in. focus to give a three- 

 fold enlargement ? By formula : 



6 -}- - = 8 inches. 



Distance of bromide paper from lens comes out 

 by formula : (N -f i) F = P. 



(3 -f i) 6 = 24 inches. 



