404 M } i<> r L - Darwin. On the Metll d f 



generally useful, and in language which could not fail to be under- 



A complete scientific investigation of a lens from every point 



of view would occupy so long a time as to make the necessary fee 



quite prohibitive, and, moreover, the results would contain much infor- 



mation which would be quite useless to the ordinary user of the lens. 



There are undoubted advantages in testing a lens by the examina- 



tion of negatives made by it, but it may be here stated, once for all, 



that the question of expense rendered it impossible, for the present, 



to adopt any photographic method ; eye observations alone have to be 



relied on. 



The form of entry is made to state for what special purposes the 

 lens is intended, whether for portrait work, for landscape views, 

 or for copying plans, &c. Every lens for photographic purposes is 

 more or less of a compromise. Great rapidity, great perfection in 

 definition, and power of covering very wide angles are incompatible 

 qualities, and one or other of them must be sacrificed. It is therefore 

 evidently unfair to expect different types of lenses to give equally 

 good results under the same test ; for if we select a lens excelling 

 greatly in one of these qualifications, we must deliberately abandon 

 the expectation of its attaining the highest standards in the others. 

 For example, in a portrait lens great rapidity is required, but, on the 

 other hand, a less high standard of definition near the edges of the 

 plate can be tolerated than with a landscape lens. No opinion could 

 possibly be expressed at Kew as to the wisdom of demanding extra 

 perfection or powers in any respect, and it is therefore necessary that 

 the lenses should be, to a certain extent, classified by the parties 

 sending them in for examination. 



The smaller the aperture of a lens, the larger will be the field of 

 sharp definition covered by it, and a complete study of a lens would 

 tell us the size of the plate which is properly covered when each of 

 the different stops is used. Considering the restrictions necessarily 

 imposed on the work, such a lengthy examination could not possibly 

 be thought of. Hence, when discussing the programme of tests to 

 be applied at Kew, it was soon evident that the time devoted to the 

 examination of each lens had to be limited by making the person 

 entering it state either the number of the largest stop by which it 

 should be judged, or the size of the plate for which it would be used ; 

 on the first supposition the Kew certificate would have given the 

 size of the plate which the lens covered satisfactorily with the named 

 stop ; and, on the second supposition, it would have indicated the 

 size of the largest stop that could be used to give results up to a 

 certain standard, or the rapidity of the lens in normal cases when 

 used for the plate of the named size. The latter of these two 

 alternatives has been adopted, because it is considered that the owner 

 or intending purchaser of the lens will, in most cases, have already 



