Examination of Photographic Lenses at Kew. 425 



accurate method of focometry, and, in the case of the image within 

 this limit being distorted, that the focal length thus obtained, even 

 though it is not identical with the principal focal length measured on 

 the axis, is what the photographer in reality wants to ascertain. The 

 Kew method is, therefore, we believe, open to no criticism on theo- 

 retical grounds as far as the value of the results is concerned. 



Under the same heading as the principal focal length is also re- 

 corded the " back focus," or the length, from the ground glass surface 

 to the nearest summit of the lenses, when the focus is adjusted on a 

 distant object. The difference between the principal focal length and 

 the back focus therefore gives the distance of the nodal point of 

 emergence from the inner summit of the lenses, thus enabling any- 

 one to mark the place where the principal plane cuts the mounting. 

 In symmetrical lenses, which are generally used for plan, work, the 

 position of the principal plane of incidence, or the point from which 

 the distance 'of the object must be measured when regulating en- 

 largements, can also be marked ; for it then occupies the same rela- 

 tive position with regard to the furthest summit of the lenses (that 

 is, to the outside end of the lens) as the nodal point of emergence 

 does to the inner summit of the lenses. 



12. Curvature of the, Field, or of the Principal Focal Surface. After 

 focussing the plate as its centre, movement necessary to bring it 



into focus for an image inches from its centre = inches. 



Ditto for an object inches from- its centre = inches. 



The following is the method of finding the curvature of the 

 principal focal surface. The image of a distant object (or of the 

 collimating telescope) is thrown on that point on the ground glass 

 where the axis of the lens cuts it, the focus is accurately adjusted, 

 and the focus scale is read off. The swinging beam is then moved 

 so that the image comes successively to positions at convenient in- 

 tervals from the centre of the plate, and on each occasion the focus is 

 adjusted afresh, and the focus scale read off. By subtracting the 

 central reading from these outer readings, the results recorded in the 

 Certificate of Examination are obtained. 



But a mere observation of the curvature of the focal surface does 

 not at once indicate how serious is the evil effect of this defect in the 

 lens. Further consideration is necessary to settle this point. If the 

 results furnished by this test are plotted in the form of a curve, they 

 will represent a section through the principal focal surface ; let AF 

 in fig. 6 be such a curve, and let cd (= /u) be the movement 

 necessary to bring the plate into focus at its margin ; let N a F be the 

 principal focal length, and EG(= e) the effective aperture of the 



