672 



Transactions of the Society. 



phenomena will be reversed, and resolution upon a dark ground 

 ought to be carried to a further point than when on a bright 

 ground ; also, it appears that when the width of the line is equal 

 to that of the interspace, it ought to be a matter of indifference 

 whether the examination be made upon a bright or a dark field. 



Experiments show that this is just what takes place. The case 

 chosen for illustration in the first instance was selected because it 



Fig. 157. 



is the one most often observed, for resolutions are seldom if ever 

 carried on with low powers. It seems, however, that with object- 

 glasses of N.A. 0*25 to 0*35, resolutions upon a dark ground are 

 about as strong as those upon a bright ground, but with object- 

 glasses of N. A. • 20, and lower, resolutions are better carried out 

 upon a dark ground. 



The following corollary is of interest. Upon a bright ground 

 no object of less size than an antipoint canbe imaged at all. Upon 

 a, dark ground the most minute object must have an image the size of 

 an antipoint. 



In conclusion, there are three practical lessons to be learnt 

 from the preceding investigations : — 



1. When periodic structures are examined with object-glasses 

 having a N.A. of 0*35 and upwards, they should be placed upon 

 a bright ground. 



2. When periodic structures are examined with object-glasses 

 having a N.A. of 0*20 and less, they should be placed upon a dark 

 ground. 



3. Minute dots, specks, flagella, etc., should be examined, when 

 possible, upon a dark ground. 



