Theories of Microscopical Vision. By A. E. Conrady. 553 



(b) That we obtain equal resolving power in all directions, 



and can therefore see simultaneously everything that 

 a given combination of condenser and objective can 

 show. 



(c) That false images, due to a badly corrected object glass, 



are not likely to deceive the observer, because such 

 objectives will not bear this mode of illumination. 



4. One other important result has been arrived at since this 

 inquiry was opened, and is, I believe, largely due to it. 



It is a warning against dark-ground illumination. In supply- 

 ing an experimental proof of the phase-reversal in diffraction- 

 spectra I also showed that with dark -ground illumination a grating 

 may be seen reversed, i.e. bright where it ought to be dark, and 

 vice versa. 



Mr. Eheinberg has shown an even more remarkable experiment 

 at the Eoyal Institution and again at the Quekett Club, viz. that 

 with dark-ground illumination we may see a grating doubled under 

 otherwise perfectly normal conditions. 



