j or Testing Camera Lenses. 



791 



sphericity over the whole of the emerging wave-front may 

 be at once observed and recorded. The indications are given 

 in terms of the only satisfactory criterion of good definition 

 which has yet been put forward, namely, freedom from 

 aberrations of wave-front. 



In favourable cases correction by retouching can be carried 

 out quite independently of any knowledge of optical theory, 

 or of the mode of design of the lens. 



There is only one of the aberrations that requires a further 

 special mention — that is Distortion. It is in order to 

 measure distortion that the deflector-mount 204 (fig. 6) has 

 been added to the apparatus. Distortion may be considered 

 as the formation of an image of one point in the object plane 

 in a position which, according to the ideal optical collineation, 

 should be occupied by the image of some other point. Ob- 

 viously, by symmetrjr, such distortion is always radial — that 

 is, on any line drawn from the point of the photographic plate 

 outwards from the axis of the lens, a point image which should 

 be formed at a point a is formed at some other point on that 

 line a\ see fig. 15. If a' is the image of A, while A' is the object 



point corresponding to a' by Gauss construction, then the angle 

 AE^A', if known, affords a measurement of the distortion at 

 this obliquit}^. This angle is in effect measured by the 

 deflector system. Let us suppose with a particular lens, at 

 obliquity U, the pencil of rays is, after passage through the 

 lens, deviated so as to cause distortion. Let us then impress 

 on the incident beam, by means of the deflector system, a 

 known deviation AU, such that the pencil of rays is once 

 more brought central with the centre of curvature of the 

 convex mirror (which centre represents the point of the 

 image plane for which the performance of the lens is for the 



