34 the president's address. 



in practical work. The order given is from the strongest to 

 the weakest resolver : — 



Appearance at Bach of Objective. 



1. Peripheral annnlus bright, 3/4 centre dark. 



2. Peripheral annulus dark, 3 '4 centre bright. 



3. The whole dark (dark ground). 



4. The whole bright (full cone). 



No. 1, which is made by placing an opaque central stop* at 

 the back of the condenser, is the strongest resolver of all 

 symmetrical systems of illumination. (This stop at the back 

 of the condenser must on no account be confused with a stop at 

 the back of the objective for the purpose of cutting out a 

 narrow central dioptric beam). It is nearly, but not quite, so 

 strong a resolver as the asymmetrical method by light in one 

 azimuth by means of a slotted stop. The resolving power of 

 No. 1 does not come under the new theory, because spectra are 

 formed in the 3,4 central portion ; the theoretical limit is there- 

 fore 96,410 times the N. A. of the objective, the wave-length being 

 the same as that used in the R. M. S. tables (line E.). No 1 

 cannot, however, be recommended for practical work, because it 

 is so liable to produce false images, and especially to double the 

 structure. Any structure near the limit for a lens of half the 

 aperture is likely to be doubled, e.g., an Angulatum, which can 

 be resolved by an objective of N.A. "7, is likely to exhibit inter- 

 costals when examined by a lens of N.A. 1*4, and illuminated 

 in this manner. This method will require a condenser whose 

 aperture must be fully equal to that of the objective. 



No. 2. — This, which is known as 3/4 cone illumination, is the 

 best for general purposes, and because of the presence of 

 spectra it also does not come under the new theory. The theo- 

 retical resolving limit for line E is 72,307 times the N.A. of the 

 objective. The image, as we have seen above, is compounded of 

 the new " true " image, and the " true false " image of the old 

 diffraction theory. It may, therefore, be relied upon. The 

 aperture of the condenser need only be 3/4 that of the objective. 



* It would be far better if the meaning of the word "stop" in micro- 

 scopical literature were coniined to the opaque central stops used at the back 

 of the condenser for producing dark grounds, etc. The common stops, 

 with central circular apertures, might be appropriately called "dia- 

 phragms." 



