428 Major L. Darwin. On the Method of 



When judging the quality of a lens by means of the results given 

 in this test the above table may also be used in the following 

 manner: Decide on the value of a (the diameter of the greatest 

 disc that will be tolerated in the image of a point), and find, from 

 the results recorded in the Certificate of Examination, the difference 

 of focus, /t, between the centre and the extreme corner of plate ; then, 

 knowing these two quantities, the table at once shows what is the 

 C.I. number of the stop that can be employed under these conditions, 

 or, in other words, with what rapidity the lens will work. 



It may also be remarked that this table gives for any part of the 

 plate, and for stops of given size, the radius () of the image of a 

 point after the plate has been removed a distance, /*, from its proper 

 focus in either direction, the movement being measured in a direc- 

 tion perpendicular to the plane of the plate. 



According to the recommendations of the International Congress, 

 lenses should generally be supplied with stops, numbered according 

 to the proposed system, in the following series : 1, 2, 4, 8, 16, 32, 

 04, &c. I should have thought that the series, 1, 2, 3, 5, 10, 20, 30, 

 60, &c., would have been more convenient for the purposes of mental 

 arithmetic ; for example, with the two last stops in this series, the 

 exposure would be the same multiple of the half minute or minute 

 that the unit of exposure is of the second. Both series have therefore 

 been included in the above table. 



The results recorded in the certificate under this heading may 

 possibly also be useful to the photographer in another way, by enabling 

 him to decide approximately what part of the ground glass he should 

 use when focussing. In fig. 6, let k be the point where the plate 

 M'F' cuts the principal focal surface when in the position which 

 has been proved to give the best general focus ; hence the image will 

 be perfectly sharp at Jfc, and conversely, if the focus is adjusted by 

 looking at the point k on the ground glass, the plate will be brought 

 to the position M'F' required ; but since Td is half cd, there is no diffi- 

 culty by interpolation or plotting to find the approximate position of 

 k for any given distance of the point c from the axis. Look in the 

 Certificate of Examination for cd, the difference of focus between the 

 centre of the plate and its margin, find in the above manner the posi- 

 tion of k, where the difference of focus from the centre is only half 

 cd, and we get the point on the ground glass which should always be 

 used when focussing with all stops, if it is desired to get the best 

 general focus. 



