475 



RESOLUTION WITH DARK-GROUND ILLUMINATION. 



By a. E. Conrady, F.R.A.S. 

 (Read March 26th, 1912.) 



Now that dark-ground illuinination has acquired new importance 

 for the observation of living bacteria, etc., some interest should 

 attach to the question of the resolving power obtainable there- 

 with. 



It is, of course, w^ell known that the chief uses of this kind 

 of illumination do not aim at, or depend upon resolving power, 

 but ow^e their value to the almost unlimited contrast which can 

 be obtained against a black background. Cases must neverthe- 

 less arise where the resolution to be expected from any given 

 combination enters as a factor, and in such cases the rules given 

 below should be of some value. 



The only theory which has been sufficiently developed to be 

 able to solve such a problem is Abbe's. Even this can only deal 

 with regular periodic structures which yield the well-known 

 diffraction spectra ; but for these the solution of the problem is 

 extremely simple, as the theory supplies the proof that resolution 

 is obtained when any two neighbouring diffraction spectra * are 

 simultaneously admitted by the microscope objective, which leads 

 at once to the postulate that the utmost possible resolving power 

 of any objective is realised when two such neighbouring spectra 

 occupy the extreme ends of a diameter of the objective. A 



* The " direct " light in this connection is to be counted as one of the 

 spectra the central one. 



JouRN. Q. M. C, Series II. No. 71. 32 



