430 Major L. Darwin. On the Method of 



after another, beginning with the largest and going on to smaller 

 ones, until the image of the black line on the bright ground is first 

 just Visible; the C.I. No. of the stop with which the lens gives defini- 

 tion up to a known standard at the extreme corner of the plate is 

 thus ascertained, and, as it may fairly be assumed that the definition 

 will be no worse than this at any other part of the plate, it follows 

 that the defining power over the whole plate comes up to or exceeds 

 the standard selected. 



It cannot be denied that the defining power is the most important 

 quality of a photographic lens for almost every purpose, and yet the 

 best method of testing definition has never been satisfactorily dis- 

 cussed or considered. If a thoroughly good test could be devised, it 

 would be hardly necessary to examine at Kew for curvature of field or 

 for astigmatism, for these defects are only hurtful in so far as they 

 affect definition. But it must be confessed that the method above 

 described is open to some objections, and the following discussion 

 is merely intended to show that it is the best that could at present 

 be devised. 



In considering this question, it was natural that attention should 

 first be turned to the excellent arrangements adopted at Kew for 

 testing the definition of telescopes. The method generally used, 

 especially when dealing with instruments supplied for the public 

 service, is to compare each one separately with a standard telescope 

 by an observation on a distant object; telescopes sent for examination 

 can by this means be passed or rejected, but hardly classified. But in 

 examining photographic lenses, where there is a much greater variety 

 of form and pattern, it would be quite out of the question to keep a 

 sufficient number of standard lenses to be of any practical use. 

 Thus little assistance was obtained from the experiences gained in 

 the examination of telescopes. 



It was necessary therefore to seek some method which did not 

 depend on comparisons with standards, and in devising such a test 

 the object most to be kept in view was evidently to diminish as far 

 as possible the errors due to the variations either in the transparency 

 of the atmosphere or in the personal qualities of the observer. 



With regard to the first point, that is, the effect of fog, mist, and 

 dust in the air, the only way to avoid errors from these causes appeared 

 to be to conduct this test in a room. This was considered especially 

 necessary in a climate like that of London. It is no doubt theoretically 

 right to examine portrait lenses, or lenses for copying plans, by 

 observations on a test-object not too far away; but for landscape 

 lenses a distant test-object would, from other points of view, be pre- 

 ferable, and the adoption of the examination in a room was only the 

 choice of the lesser of two evils. 

 With regard to variations due to the personality of the observer, the 



