Examination of Photographic Lenses at Kew. 421 



emergence for visual rays should be the same as the nodal point for 

 actinic rays. 



In no lens are these conditions perfectly fulfilled, but before dis- 

 cussing the nature of the errors thus introduced it may be as well to 

 consider shortly for what purposes and with what degree of accuracy 

 the practical photographer wants to know the focal length of his 

 lens. Two uses to which this knowledge would or could be put have 

 already been mentioned, and we know of no others. In the first 

 place, it has been shown how the numbering of the stops depends on 

 the focal length, and how advantageous is the knowledge of the 

 intensity of the illumination of the plate which may thus be gained. 

 But as, on account of the difference in the amount of reflection and 

 absorption of the lenses, two lenses with the same C.I. number of 

 stop may differ more than 10 per cent, in the intensity of illumination 

 in the centre of the field, as in the same objective the difference of 

 illumination of different parts of the field is generally more than 

 20 per cent., and as the photographer is seldom able to estimate his 

 unit of exposure within this latter percentage, it can hardly be 

 seriously contended that the focal length must be known with very 

 great accuracy for this purpose. The second object for which the 

 photographer may require to know the focal length is for the use of 

 the tables in which the distance is given at which the object has to 

 be placed to obtain a given enlargement or reduction ; it has already 

 been stated that this is not, we believe, a want often felt, except for 

 getting approximate results ; but if the focal length is used for final 

 adjustments in this manner, it should be known with very consider- 

 able accuracy. 



With regard to the first condition, as to the focal surface being a 

 plane, it should first be stated that it is found convenient at Kew to 

 bring the ground glass into focus when the swinging beam is in con- 

 tact with one of the stops, thus insuring the greatest sharpness of 

 image at the points of observation ; that is to say, in fig. 4, the prin- 

 cipal focal surface is made to pass through the points B' and C', and, 

 if it is not a plane, it may be represented by the dotted curve C'F 2 B'. 

 Under these circumstances, therefore, the principal focus will be at 

 F 2 , and N"iF 2 will represent the principal focal length according to 

 our definition ; but it has been shown that the observation gives N X F 

 as the focal length, thus introducing an error equal to FF 2 in the 

 result. It is to be observed, however, that with a lens giving a 

 markedly curved focal surface, the photographer, in order to get a 

 general minimum amount of diffusion, would adjust his focus by 

 looking at the image at a point somewhat more than half way from 

 the centre to the margin of his plate: for example, with a lens 

 covering 50 or 60, he would focus at a point some 15 from the 

 centre, or at about the position where the Kew observation for the 



VOL. LII. 2 F 



