106 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
2. ih. X 3000. 
3. ih. X 8000, upper part of the valve, showing square beads identical 
with those of Amphipleura LindJieimeri Grun. 
4. Ampliipleura Lindlieimeri Grun. x 2500. 
5. Surirella gemma Ehr., about X lOUO. 
6. Pleurosigma angulatum, in hexagons, about X 10,000. 
7. Van Heurclda crastinervis Bieb. (Frusiulia saxonica Eabh.) 
X 2 00. 
8. Van Heurclda crasslnervis, Breb., about X 6000. 
In all the photographs the focus was upon the intermediate layer, 
and here and there in most of them the gradations of form are shown 
between squares and hexagons. 
Plate III. — Pleurosigma angulatum W. Sm. x 2000. On the right of 
the centre the illusory intermediate beads are seen at the same time as 
the real beads (the openings), of hexagonal form. 
The photographs were all produced with Zeiss’s new apochromatic 
1/1 0 in. of 1 • 63 N.A. Monochromatic sunlight. Compensating eye-piece 
(special) 12. Condenser 1*6 N.A. 
The preparations were all in a medium of 2*4. Cover-glass and 
slides of flint, 1*72. Diatoms melted into the cover-glass softened by 
heat. 
Ilford dry plates, developed with hydroquinone and eosine solution 
as suj^plied by Mercier, of Paris. 
Resolving Power a “ Superfetation.” — The following extract from 
M. A. Zune’s ‘ Traite de Microscopic’ (1889) should be interesting to 
microscopists. 
“ Resolving power. We regret not to have the necessary authority 
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, fibrillae, 
depressions, reliefs, &c. ; but an objective which 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 commented 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 have to repeat again, an objective may have perfect 
defining power, and yet, by reason of its want of aperture, it will be 
unable to show particular markings. It defines all that it can take up, 
but cannot define what is not imaged by it. 
It would be possible, no doubt, to arrange that “ 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 tlie two qualities, an objective 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.” 
