for Testing Camera Lenses. 



789 



Now let us consider the successive strips of the lens corre- 

 sponding to the regions bounded by these finest lines. . The 

 light passing through the first complete strip ma}?- be con- 

 sidered to have the phase 0, the second strip the phase O'l w, 

 the third strip the phase 0*2 ir, and so on. The total dis- 

 turbance at the centre of the reference sphere, provided the 

 wave-front travelled on, can be approximately found in the 

 following way. The enlarged photograph is cut into strips 

 along the finest lines. Each strip is weighed, and a drawing 



Fig. 13. 



0-4 



0-3 



0*2 



made in which the weights of the strips are vectorially added, 

 the direction of each vector being that indicated by the phase 

 relationship. The amplitude of the resultant disturbance 

 will of course be represented (see fig. 14) by the resultant 

 vector OM, and the relative intensity of illumination at that 

 point by the scalar square of this vector. 



One can carry this process out with a much greater 

 speed than the corresponding mathematical calculation, even 

 assuming we could easily express the shape of wave-front 

 obtained experimentally in a convenient mathematical norm, 



