432 



Major L. Darwin. On the Method of 



The result of bad definition in the lens is to make the image of a 

 point occupy a sensible area on the photographic plate, and conse- 

 quently to prevent the image of the edge of a surface from being 

 sharply indicated. The general effect can be best illustrated by 

 means of figs. 7 and S, where the abscissae are enlarged dimensions 

 measured on the plate, and the ordinates indicate the intensity of 

 illumination at each point. In fig. 7 let a, e, b represent a section 

 through the image of a small spot of light. In fig. 8 let the curve 

 /, /?, k represent the actual image of the edge of a bright surface, 

 which would be represented by/, d, c, k if the defining power of the 

 lens were perfect ; it is evident that a b' in fig. 8 is equal to the 

 limiting value of ab in fig. 7 as the spot of light becomes infinitely 



FIG. 8. 



FIG. 7. 



FIG. 9. 



FIGS. 10 AND 11. 



