436 Major L. Darwin. On the Method of 



14 Distortion. Deflection or sag in the image of a straight line which, 

 if there were no distortion, would run from corner to corner along 



the longest side of a ly plate = 0' inch. 



The following is the method adopted at Kew of measuring the 

 distortion produced in the image by the lens under examination. 

 Lefc fig. 14 be a vertical section through the testing camera ; GGr re- 



FIG. 14. 



presenting the ground glass ; F the principal focus ; and N^ the 

 horizontal axis, which passes through the nodal point of emergence, 

 the adjustment for that purpose having already been made for test 

 No. 10. The lens-holder carrying the lens is first turned in either 

 direction through an angle /3, such that PF, or FNi tan /3, or / tan /3 

 is equal to half the shortest side of the plate for which the lens is 

 being tested. (The horizontal movement of the swinging beam in 

 the testing camera gives an easy means of determining the angle /3 ; 

 a distant object is first brought to focus at the centre of the ground 

 glass, and then the swinging beam is revolved about the axis A (see 

 fig. No. 1) until the image has moved along the graduated scale a 

 distance equal to half the shortest side of the plate ; the beam is thus 

 made to move through the angle p. which can be read off with 

 sufficient accuracy on BC, the top of the wooden stool, which is 

 graduated for that purpose). After this adjustment has been made, 



