ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
249 
of Kocli in the rival department of investigation of the infinitely little, 
to determine what it can tell us in regard to the structure and growth 
of bacteria. 
The photographs which illustrate Dr. Van Heurck’s latest work 
with the new lens seem to show two things : first, that he exhibits the 
areolation on the valve of Amphipleura with a distinctness of definite 
resolution beyond anything heretofore published ; second, that in regard 
to the less finely marked shells, no perceptible advance upon work done 
with glasses of narrower angle is apparent. A word further as to each 
of these points. 
In the resolution of Amphipleura , as in regard to other tests, there 
has been a regular progress, partly dependent on real improvement in 
lenses, and partly upon the use of better methods of manipulation. 
The mounting of the specimen has also been an element of no small 
importance. Everybody knows that a diatom mounted dry is much 
more boldly visible than one mounted in balsam. Striae are shown, in 
this case, with much less oblique light, and may be resolved by a glass 
which quite breaks down when used on the balsam mounted specimen. 
This is consistent with the fundamental principle that the angle of 
aperture of a lens determines the possibilities of its work in the resolu- 
tion of fine details in all microscopic objects. A well corrected glass 
will do more than a badly corrected one made on the same formula and 
with the same aperture. No glass of high power has ever been made so 
perfect as to perform all the theoretic possibilities for a glass of its 
angle. But however well made the lens may be, it is a mere waste of 
time and eye-sight to try to make it show details too fine for the theo- 
retic capability of its angle of aperture. 
The average fineness of striation of Amphipleura pellucida is for the 
transverse lines about 90,000 to the inch, and of- the longitudinal (by 
Dr. Yan Heurck’s measurement) about 125,000. But the finest of these 
are (by the R. M. S. tables) theoretically resolvable in photography by 
a dry glass of 180° aperture, by a water-immersion glass of 100° water 
angle, or by a homogeneous immersion glass of 83° balsam angle, all 
being of a numerical aperture 1, substantially. It thus appears that 
the angle of 1*63 N.A. is *63 in excess of what is theoretically required 
to do the work which Dr. Yan Heurck has accomplished with it, or, in 
other words, that our high power glasses do less than two-thirds of what 
perfect glasses of their aperture might do, if the tables are correct. 
Here, then, is a large margin for the improvement of the whole series of 
immersion lenses, since almost any lenses of first class makers found 
in the market, have angle enough, in the high powers, to do all that the 
new apochromatic has done. 
But before we can decide how far the new lens is superior to older 
ones of less aperture, we must have the latter tested under equal 
conditions, and it is to be hoped that Dr. Yan Heurck will add this to 
his useful labours. The new photographs are from objects mounted in 
a medium of refractive index 2 ■ 4, with both slide and cover-glass closely 
approximating the same index. They are also taken with monochro- 
matic sunlight. Ever since Prof. H. L. Smith introduced the highly 
refractive media, it has been well known to microscopists that a very 
thin and finely areolated shell like Amphipleura pellucida is so much 
