192 The Microscope. 



Editor The Microscope : — 



The February number of the Journal of the Royal Micro- 

 scopical Society, London, has the following extract from M. A. 

 Lune's Traite de Microscopic, 1889 : 



" resolving power a supkrfetatiox. 



Resolving power. We regret not to have the necessary au- 

 thority to erase this word from the dictionary of microscopists, 

 since it appears to us to constitute an entire superfetation. To- 

 say of an objective that it has resolving power is, according to 

 most authors, to attribute to it the power of isolating, so to say^ 

 one from another the finest details of structure on the surface of 

 a transparent object, such as striae, fibrillse, depressions, reliefs,, 

 etc.; but an objective w^hich defines well in the complete sense 

 of the word, ought it not to resolve perfectly ? 



This carries a long way further the error on which we com- 

 mented in the case of the Quekett discussions, where, however, 

 it was not proposed to abolish the term ' resolving power ! ' As 

 we explained then, and shall probably haye to repeat again, an 

 objective may have perfect defining power, and yet, by reason of 

 the want of aperture, it will be unable to show particular mark- 

 ings. It defines all that it can take up, but can not define what 

 is not imaged by it. 



It would be possible, no doubt, to arrange that the definition 

 should be considered to include ' resolving power,' but nothing 

 would be gained by confusing the two terms, especially as we 

 have already the term suggested by Prof Abbe — delineating 

 power — to denote the combination of the two qualities, an ob- 

 jective having large delineating power when it both defines well 

 and has large aperture. 



The author's views are in other respects peculiar, as he is of 

 opinion that ' an objective of large angle, well constructed, will — 

 all other things being equal — show details in depth as well as it 

 will show those on the surface.' " 



Thinking that you may not have seen and published thig 

 communication, and that it will be interesting to you and other- 

 microscopists, you receive this copy from. 



Yours very truly, 



Dover, N. H. Dr. Carl H. Horsch. 



