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II. — An Easy^ and Sim]de Method of Besolving the Finest-lined 
Balsamed Diatomaceous Tests hy transmitted Lam'plight, ivith 
special reference to Amphipleura pellucida. By Adolf 
Schulze, Glasgow, 
(^Read before the Pioyal Microscopical Society, March 6, 1878.) 
The resolution of the finest-lined tests, and especially of the mark- 
ings of Ampthipleura pellucida, is at all times a matter of some 
difficulty, and requires for its accomplishment the best objectives 
and the most careful arrangement of the illumination. It is gene- 
rally assumed that the resolution of the closest markings on diatoms 
is far more difficult when they are mounted in balsam than when 
they are mounted dry, and with the ordinary means of illumination 
this appears to be indeed the case ; but I purpose to describe a 
simple method by which even so small and closely striated speci- 
mens as Nos. 18, 19 and 20 on Moller’s Diatom- Probeplatte, and 
other still finer valves, having about 100,000 lines per inch, may 
be easily and unmistakably resolved into distinct striae by lamp- 
light, and which by sunlight yields so excellent results that I can 
only compare them to those admirable photographs of Amphi- 
pleura pellucida taken by Dr. Woodward. It must, however, be 
borne in mind that the ridges constituting the markings of these 
minute diatoms mounted in balsam, possess a minimum of substance, 
and being extremely transparent, cannot throw hlach shadows such 
as those of the larger and coarser frustules, and that if the field 
were too brilliantly illuminated these infinitesimal shadows would 
be totally obliterated. Some people may be inclined to think that 
the markings shown as alternate grey and white lines are only 
faintly resolved; but microscopists whose eyes are cultured by 
observations on diatoms will readily admit such a resolution to be 
perfect, provided no spurious or coloured lines make their appear- 
ance, and they will grant that this is all which can be expected to 
be seen under the most favourable circumstances. 
The best dry achromatic condensers, and even Powell and 
Lealand’s supplementary stage and small plano-convex condenser, 
fail, as far as I know, to aid in the resolution of balsamed specimens 
of A. pellucida ; and those few observers who have succeeded on 
this difficult test by lamplight have not published any accounts of 
their modes of illumination. 
In an article on the Immersion Paraboloid and the Eeflex 
Illuminator, which appeared in the ‘English Mechanic’ of 7th 
December, 1877, Mr. F. H. Wenham claimed as one advantage of 
the latter over the former that it would give a dark ground on 
balsamed objects even when they were viewed with immersion 
