442 PROCEEDINGS OF THE SOCIETY. 



image of Ramsclen's disk which they form would ultimately require higher 

 magnificatio]! to ])ring the disk up to the required scale for measure- 

 ment, this again causing a loss of light and possibly of definition as 

 well. In the work on which Mr. Hartridge was engaged, one of the 

 chief difficulties was to make the diffraction spectra sufficiently bright 

 for measurements to be made on them ; it was therefore necessary to lose 

 as little light as possible. 



A further point of importance is the fact that Mr. Hartridge's 

 method is independent of the tube-length and magnification used with 

 any given objective ; this, combined with the presence of the straight- 

 line relationship between the scale of the micrometer and the numerical 

 aperture, allowed the observer to check from time to time the accuracy 

 of his measurements against a standard apertometer. The straight-line 

 relationship is due to the obedience by the objective of the sine 

 condition. 



Mr. Hartridge then read his second paper on " A Method of In- 

 vestigating Diatom Structure." 



On the conclusion of the paper, the President said he thought that 

 those present would divide themselves into groups — those who, like him- 

 self (the minority) were abysmally ignorant of a very difficult subject ; a 

 second group, including those who knew more, were curious to know still 

 more ; and a third, composed of those who were skilled in the matter 

 and would criticize Mr. Hartridge's methods of carrying out his investi- 

 gations. He should like to say, however, how much he admired Mr. 

 Hartridge's ingenuity and pertinacity and the very great skill he had 

 shown in adapting his means to his various ends. 



Dr. Shillington Scales opened the discussion by reading the second 

 portion of Mr. Gordon's letter dealing with Mr. Hartridge's further 

 communication : — 



" With regard to the investigation of diatom structure, the method 

 to which Mr. Hartridge calls attention is one which has, I think, many 

 valuable applications : but I must confess to a suspicion that the in- 

 vestigation of diatom structure is one of the least profitable of the fields 

 into which it can be carried. Mr. Hartridge's experiments show how 

 great are the difficulties that have to be faced, and a little further con- 

 sideration of the matter, shows that when these difficulties have been 

 successfully surmounted the results obtainable must be very restricted 

 in quantity, and often very difficult of interpretation. Mr. Hartridge 

 has worked upon a well-known diatom exhibiting extraordinary regularity 

 of structure, and therefore it is easy, in the first place, to obtain a dif- 

 fraction pattern and, in the second place, to interpret it. But in a large 

 number, I should imagine in the case of the great majority of diatoms 

 no definite diffraction pattern is obtainable. I may take for instance 

 Surirella as an example, where there is plenty of structure but so little 

 regularity of formation that no intelligible or analysable diffraction 

 pattern can be obtained from it. Then, even in the case of a well 



