Examination of Photographic Lenses at Kew. 407 



The O.I. No. of a stop means the number which indicates the 

 intensity of illumination produced by it on the plate according to 

 the system proposed at the International Photographic Congress of 

 1889 (see p. 413). 



The largest normal stop means the largest stop that can be used 

 with the lens so as to produce definition up to a selected standard of 

 excellence all over a plate of given size, the objects whose images are 

 seen being all equally distant. 



A slow lens means one of which the largest normal stop has a less 

 diameter than has C.I. No. 6. 



A moderately rapid lens is one of which the largest normal stop is 

 C.I. No. 6, or larger than that size and less than C.I. No. 2. 



A rapid lens is one of which the largest normal stop is C.I. No. 2, 

 or larger than that size and less than C.I. No. 2/3. 



An extra rapid lens is one of which the largest normal stop is C.I. 

 No. 2/3, or larger than that size. 



For convenience of reference, these definitions will in future ac- 

 company the certificate, probably in the form of additional notes. 



No doubt most lenses are supplied with stops larger than the ones 

 here called the largest normal stops, even if it is not intended to use 

 smaller plates than those under consideration ; this is, of course, very 

 right, for in many cases the photographer will be willing to sacrifice 

 the definition near the edge of the plate for the sake of increased 

 rapidity. 



It now remains to be shown in what way the above certificate of 

 examination would be useful to the practical photographer, who has 

 sent his lens to Kew for the purpose of being tested. It may, we 

 think, v be assumed that he wants answers to the three following 

 questions : 1st. Is the lens a good one ? 2ndly. Does it properly 

 cover the plate of the named size ? And 3rdly. What exposure 

 must be given when using the different stops ? 



With regard to the two first questions, the result of the examina- 

 tion is recorded in such a way that he may either rely on the 

 " General Remarks," or he may form an independent judgment from 

 the results of the tests. 



In order to decide himself, from the records in the certificate, 

 whether the lens is generally speaking a good one, he should first 

 look to test No. 13 to see if the definition in the centre of the plate 

 with the largest stop, is " excellent," as should always be the case ; 

 he should then consider test No. 15, by which he will see what are 

 the faults introduced by the lens not being properly corrected for 

 chromatic aberration. With regard to the second question, that is 

 to say, when considering whether the plate he intends to use is 

 properly covered or not, he should chiefly look to the results recorded 

 under test No. 13, where is given the size of stop or the rapidity of 



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