790 Mr. F. Twyman on an Interferometer 



and by working from point to point by means of the deflector 

 system the intensity distribution in the focal region can be 

 completely explored. 



Fig. 14. 



It is necessary in the design of photographic lenses to con- 

 sider the above aberrations and their effect as a whole upon 

 the focus: for obviously it is futile to design a lens that pro- 

 duces images showing an absolute freedom from one particular 

 tyoe of aberration, if other aberrations are present to such a 

 degree as to render the lens useless. It therefore follows 

 that the residual aberrations must be so adjusted that the 

 focus, although affected by minute quantities of aberration, 

 does not betray the presence of any one aberration to an 

 extent that is likely seriously to interfere with the efficient 

 performance of the lens ; and herein the ability of the 

 designer manifests itself. This done, there can be no better 

 verification or criticism of the design of a photographic lens 

 than that yielded by a thorough test on the Camera Lens 

 Interferometer, since the extent of the departure from 



