\-2-2 Major L. Darwin. On the Method of 



focal length is taken; thus, with such a lens, C'B', in fig. 4, would 

 represent the position of the photographic plate ; and it is evident 

 that, for all questions of illumination or enlargement, NjF, or the 

 distance from the plate to the point from which all the light may be 

 supposed to emanate, should be introduced into the calculations, and 

 will give the true results, or, at all events, more nearly the truth than 

 if NiF 2 , the true principal focal length, had been used in its place. 

 Thus, by recording the length N"iF in the Certificate of Examination, 

 we always give more nearly what the photographer practically wants 

 than if the length N\F 2 , or the true principal focal length, had been 

 ascertained. But in any case the point raised in this paragraph could, 

 if thought desirable, be met by focussing the plate in the centre of 

 the field when the observation for focal length is made. 



The second point raised, as to the theoretical correctness of the 

 principal focal length as found at Kew, is with regard to the distor- 

 tion of the image, which may be described as the results due to the 

 theory of the nodal points being not strictly applicable except near 



FIG. 5. 



the .centre of the field. In fig. 5, let N, and N 2 be the nodal points, 

 e prmcipal focns and B" and C" the images of the infinitely 



stout po,n . B and C ; if there is distortion, the lines SB" and SC<^ 

 Irawn parallel to the incident rays, do not cut the axis at N,, the 



oaai point pt emergence: let these HTI<.= f ^ i, *!,. L c< _ L -!, 



f simiiitude 



, COT1Stmctl " represents the Kew method of 

 n, and therefore SF is the distance found as the principal 



