Examination of Photographic Senses at Kew. 

 FIG. 17. 



439 



FIG. 18. 



at / and li respectively ; ef and gJi will, therefore, be the total dis- 

 tortion in each case. In fig. 18, let the rays coming from the objects, 

 of which the images are seen at e and g, make the angles ft and 

 with the axis of the lens at the nodal point ; if ef and eg are equal 

 in length to the lines similarly denoted in fig. 17, it is evident that 

 the curve ege represents the image of a straight line, which, if 

 there had been no distortion, would have appeared as the line fhf. 

 Now it would not have been difficult to have devised means of 

 measuring the total distortion at any part of the plate; for in- 

 stance, to have measured the distortion ef for the point e at the 

 corner of the plate but the following considerations show, it is 

 thought, that that would not be a suitable way of testing the 

 lens ; let the curve efe in fig. 18 represent the greatest curvature 

 that would be tolerated for the class of work for- which the lens is 

 intended; compare the lens producing this curve with another in 

 which S/s occupies the same position, but in which S$ is nearer the 



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