408 



Major L. Darwin. On the Method of 



the lens for a given standard of definition up to the edge of the plate ; 

 if the definition at the centre is " excellent," then any want of defini- 

 tion at the margin will be chiefly due to curvature of the focal surface, 

 or to astigmatism, and therefore the results of tests Nos. 12 and 16 

 should be considered at the same time as test No. 13. He must also 

 look carefully to the result of test No. 14, which shows the maximum 

 distortion produced in the image; it will depend for what class 

 of work the lens is to be used whether he should consider the 

 amount of curvature in the image of a straight line near the edge of 

 the plate, which will be there indicated, is objectionable or not. 



The " General Remarks " are recorded as the result of exactly 

 similar considerations to those discussed above, the experience gained 

 by the examination of lenses of undoubted quality giving an idea of 

 what standard of excellence should be required. 



With regard to the third question, as to the exposure to be given 

 with the different stops, it may be hoped before long that the O.I. 

 numbering will be generally adopted by all practical photographers, 

 in which case the results of test No. 9 will give the information 

 required. 



In many works on photography the view is expressed that the 

 practical photographer also wishes to know from what point on his 

 lens he should measure or adjust the distance of any object so that 

 by reference to tables, he can obtain definite enlargements or reduc- 

 tions ; this is, in fact, urging that the position of the principal planes 

 should be marked on the mounting of all lenses. According to our 

 experience this is a want in reality very seldom felt in practice. 

 The tables are, no doubt, sometimes used to get approximate results, 

 the fine adjustment of scale being afterwards done by measurements 

 on the ground glass ; bat if the slot between the two lenses of a 

 doublet is used as the point from which the measurements of distance 

 are made, the results will be quite near enough to the truth to serve 

 as a first adjustment, and for this purpose nothing will be gained by 

 marking the exact position of the principal planes ; it should, 

 however, be stated that the omission to mark them is merely made in 

 consequence of the necessity felt of minimising in every possible 

 direction the time spent in the examination. 



Each test to which the lens is subjected will now be described in 

 detail, together with such discussion as to the reason for its adoption 

 as may appear necessary. 



The first four headings of the certificate deal with the numbering 

 of the lens, the maker's name, the size of plate for which the lens is 

 to be examined, Ac. ; and, as these do not form part of the results of 

 the examination, no remarks are necessary with regard to them. 



