Examination of Photographic Lenses at Kew. 413 



effective aperture of the largest stop, as recorded in the Kew Certi- 

 ficate of Examination. The ratio of the effective aperture to the 

 diameter is the same for all stops of the same lens, and the effective 

 aperture of the other stops is either measured as above, or calculated 

 from the ratio thus found. As the rays are parallel when emerging 

 from the lens, it is evident that, if the stop is in front of all the lenses, 

 the effective aperture will be the same as the diameter of the stop 

 itself. 



By imagining the path of the rays in the above experiment as 

 being reversed, in which case the rays forming the cylinder are all 

 brought to a focus on the plate, it is evident that the intensity of 

 illumination of the plate at the centre, when focussed for distant 

 objects, varies directly as the sectional area of the cylinder, and 

 therefore as the square of the effective aperture as above measured. 

 The intensity of illumination of the plate also varies inversely as the 

 square of the distance from the point in the lens from which all the 

 light may be supposed to emanate, that is, from the nodal point of 

 emergence. If we adopt as our definition of the principal focal 

 length (/) of the lens the length between the principal focus and 

 the nodal point of emergence, it is then evident that the square of the 

 effective aperture divided by/ 2 will be a measure of the illumination 

 of the plate. In consequence of this fact, it has for a long time been 

 customary to speak of the diameter of stops in terms of the ratio of 

 their effective apertures to the focal length of the lens ; for example, 

 a lens having a stop with an effective aperture equal to one-tenth of 

 its principal focal length is commonly spoken of as working with an 

 intensity of //10. But it has recently been found by photographers that 

 it is practically useful to adopt a definite standard or unit of intensity 

 of illumination in order that the different stops may be numbered in 

 such a way as to readily indicate the different exposures required 

 with each ; many systems of this kind have been considered : //100, 

 //10, //4, and//<v/10, each having been at various times proposed as 

 the basis of enumeration, the numbering of the stops sometimes 

 increasing and sometimes diminishing as the necessary exposure 

 increases. Each of these systems has met with considerable oppo- 

 sition from different quarters; but this is not the place to discuss 

 their relative merits, more especially as in selecting one of them for 

 the Kew certificates, we have been chiefly influenced by considering 

 which has received the sanction of the most authoritative body, and 

 which, therefore, stands the best chance of universal adoption. Judged 

 by this standard, there can be no doubt that the recomendations of 

 the International Photographic Congress of Paris of 1889, as endorsed 

 by the Congress at Brussels, should be adhered to as far as possible. 



The following system, which we have called the C.I. system, has 

 therefore been adopted at Kew. The stop//10, the effective aperture 



