Examination of Photographic Lenses at Kew. 



437 



the ground glass is brought into focus by observing the image of a 

 distant object at a point P, a little below C, the line engraved on 

 the glass ; under these circumstances, if the principal focal surface is 

 a plane, and if the lens were being used in the ordinary manner, PP' 

 would be the position occupied by the photographic plate, the section 

 shown being taken across the centre of the plate parallel to its 

 shortest side. The small distance PC is carefully measured; this 

 length is then multiplied by secant (3, thus obtaining C'P, which 

 we will call a. The swinging beam is now revolved about the pivot 

 in either direction, so that the image moves .along the scale on the 

 ground glass a distance equal to half the longest side of the plate for 

 which the lens is being examined ; the sketch in fig. 7 is still more or 

 less applicable, PP' still representing a section across where the 

 photographic plate ought to be, but this time at the end of the plate, 

 not at its centre ; (F, therefore, no longer represents the principal 

 focus) ; in fact, what has been done is to make the image describe 

 what, neglecting distortion, would be a straight line from the centre 

 to the corner along fche longest edge of the plate : after this movement 

 has been made, the length of C'P is again obtained by measurement 

 and calculation, and this time let the result be called 6 ; the operation 

 is repeated when the swinging beam is revolved to an equal angle <Dn 

 the other side of zero, and a third length, c, is thus obtained. In 

 fig. 15, let BAG be equal in length to the longest side of the 



FIG. 15. 



and let a, 5, and c be the lengths just obtained ; then the curve bac 

 will evidently represent the image of a straight line thrown by the 

 lens under examination along the edge of the longest side of the plate. 

 Since the image travels along a line , very nearly parallel to ike 

 engraved line on the ground glass, BAC will be nearly parallel to the 



chord of the curve, and -a, which is the length, recorded in the 



B 



Kew certificate, will be a very close approximation to the sagitta or 

 sag of the curve. 



The image of a rectangle near the limits of a photographic plate 

 will appear, when any distortion is visible, like one or other of the 

 forms indicated in fig. 16. The sagitta is conventionally considered 



VOL. LII. 2 G 



