Examination of Photographic Senses at Kew. 419 



cipal focal length of the lens, it may be as well to give a rigid defini- 

 tion of -what is here meant by that expression, as it has often been 

 used in somewhat different significations. The definition here adopted 

 of the principal focal length is the distance between the principal 

 focus for visual rays (or the image as seen by the eye of an infinitely 

 distant point on the axis of the lens) and the nodal point of emergence. 

 The use of the term nodal point is, perhaps, open to criticism ; under 

 the ordinary circumstances of a photographic lens the nodal points 

 and the principal points occupy the same positions, and therefore 

 either of these expressions might have been used in the definition ; 

 but if we take into consideration any imaginary circumstances when 

 these two points would not be identical, as, for instance, if one end of 

 the lens was immersed in water, it will be observed that the Kew 

 method of determining the principal focal length would find the dis- 

 tance between the nodal point and the ground glass, and not that 

 from the principal point ; moreover, under these imaginary condi- 

 tions, it would be the distance of the nodal point from the plate which 

 would chiefly be of value to the aquatic photographer, for the in- 

 tensity of illumination of his plate would vary as the square of that 

 distance and not of the distance from the principal point. But it 

 must be confessed that the term was, in reality, adopted because it is 

 that best known in the photographic world, and not on account of 

 such hair-splitting reasons as these. 



It now remains to be seen if the Kew method does give the true 

 principal focal length according to the above definition. In fig. 4, let 



FIG. 4. 



B, A, and C be three very distant points, A being on the axis of the 

 lens, and B and C being at equal angular distances on either side of 

 it ; let NI and N" 2 be the nodal points ; let C', F, and B' be the images 



