438 Major L. Darwin. On the Method of 



Fio. 16. 



Positive 

 distortion 



positive if it is measured towards the centre of the plate from the 

 chord, thus giving the name of positive and negative distortion in 

 the two cases. 



The distortion for distant objects is not necessarily exactly the 

 same as for nearer ones, and therefore the uses for which the lens is 

 intended should not he forgotten ; for example, with portrait lenses, 

 an object some 10 to 20 ft. away should be used to throw the image 

 in the above test. 



Probably it will not at once be admitted that this is the best means 

 of measuring distortion ; for no doubt it might be done in many other 

 ways, and a method might easily have been selected which would 

 have been less open to criticism on purely scientific grounds. We 

 believe, however, the Kew certificate gives the information really 

 required in practice. In order to determine if a lens is suitable for 

 any particular purpose, all that is required to be known is whether 

 the image of a straight line near the edge of the plate will show too 

 much curvature, the amount of tolerance depending greatly on the 

 work for which the lens is to be used. There is no means of enabling 

 the photographer to form a judgment on this point more readily than by 

 giving him the sagitta or sag in the image of a straight line along the. 

 edge of his plate. That it would be difficult to find a better method 

 may, perhaps, be made more evident with the aid of figs. 17 and 18, 

 the former representing a section through a lens and the photographic 

 plate, and the latter showing part of the plate in plan, with the curved 

 image of a straight line just inside its margin. In fig. 17, let N\ be 

 the nodal point of emergence ; 8/3 the centre of similitude for rays 

 emanating from a distant object and making an angle (3 with the 

 axis ; and S* the same for an object at an angle ; e and g will, 

 therefore, be the images of these two objects as seen on the plate, 

 whereas, if there had been no distortion, they would have appeared 



