250 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
more easily resolved in them that the test when so mounted loses very 
much of its difficulty. If we make the slide and cover-glass nearly or 
quite homogeneous with the medium, and in addition to this increase 
considerably the aperture of the substage condenser, and connect it with 
the slide by a highly refractive immersion medium, it needs no telling 
that the difficulties of resolution have been still further and very greatly 
diminished. Prof. Abbe and Dr. Van Heurck deserve great praise for 
devising and putting to use the means of effecting such favourable 
conditions ; but the difference between these and the conditions under 
which other glasses are used must be eliminated before we can tell how 
much of the improvement in work is due to the lens, and how much to 
the conditions. 
In regard to my second point, viz. that as to the less finely marked 
shells, no perceptible advance in the character of work is apparent, I will 
only say that Dr. Woodward’s photographs, exhibited at the Centennial 
Exposition at Philadelphia in 1876, must remain the standard of com- 
parison when the work of the older lenses is brought into question. He 
worked with a Powell and Lealand water-immersion of 1/16 in. of 1869, 
and Tolles’s 1/10 and 1/18 of similar construction. A little later he 
used also Spencer’s glycerin immersion 1/6 and 1/10, Zeiss’s homo- 
geneous immersion 1/8 and 1/12, and a homogeneous 1/10 by Tolies. 
I think none of these glasses had an aperture greater than 1*25 N.A. 
Whilst writing this paper I have examined those photographs afresh, 
and am entirely sure that for the exhibition of the areolation and struc- 
ture of Navicula rhomboides, both the coarser and finer forms (including 
Frustulia saxonica or Navicula crassinervis ) Surirella gemma , AmpMpleura 
Lindheimerii, Pleurosigma angulatum, and for the transverse striation of 
Amphipleura pellucida, they are fully equal to anything that has been 
done with the most recent and widest angled glasses, not merely as 
photographs, but as conclusive evidence of the quality of the glasses he 
used and their satisfactory work within the limits named. 
Dr. Van Heurck has taken one step in advance (and it is a real one), 
which shows with what labour each step is now gained. The new glass 
has cost Prof. Abbe months of labour, as is reported ; and no incon- 
siderable expense in money, as well as time, has been lavished upon it. 
The result, to put it in its most general form, is that where we could 
distinguish objects in the approximate form of circles or squares of a 
diameter of 1/100,000 in., we may now (under exceptional conditions) 
distinguish them if 125,000 to the inch. Yet this may make the 
difference between tracing definitely some part of the life-history of a 
bacillus or failing to trace it. 
The apochromatic system is by no means synonymous with increase 
of angular aperture, though it adapts itself readily to the widening of 
angles. It is distinctively a step in the reduction of the conflict between 
the chromatic and spherical corrections, by the aid of the wider range 
in refraction and dispersion which the newly-invented Jena glass pos- 
sesses. It is therefore directly aimed at the problem stated before, viz. 
the bringing of the practical performance of our lenses more nearly to 
the standard of their theoretic possibilities. The effort to do this by 
using material of higher refractive index for lenses is an old one. Even 
diamonds have been used for experiment in this direction. The solution 
