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TERMINOLOGY AND CLASSIFICATION OF DIATOMS. 89
screening oneself from reflected light and giving several quick
turns to
the fine adjustment screw, the variations in the luminous intensity of
the object enable very
calculated.
minute differences of refractive index to be
It is indeed easy to calculate a difference of o.ooi without any ambiguity.
To obtain a good measurement, it is well to use monochromatic light, the
dispersive power of the liquid never being the same as that of the
solid. From this it follows that it would always be known,
at least
to about 0.001 whether a crystal has a greater or less index of refraction
than that of any given liquid. This is to a large extent sufficient to
diagnose a mineral.
The following will shew how Mr. Brun's method can be applied
to prove that the beads on diatoms are alveoles :
A large species of Coscinodiscus is chosen and examined in pure
water. The beads are then seen to have a less refractive index than
that of the silicious skeleton, for on raising the tube of the microscope
they are seen to have a dark centre.
The same species examined in styrax shews that the refractive index
of the bead is greater than that of the silicious skeleton, for by raising
the tube they are seen to have a brilliant centre.
This double experiment proves, therefore, that the bead always
assumes the refractive index of the medium in which the diatom itself
is placed ; now if the bead were solid this could not be the case, and
consequently it follows that the bead must be a cavity. Even as far back
as 1885, in my "Synopsis,"
I had arrived at the conclusion that alveoles
existed, among other reasons because of the appearance that the valves
assumed in the yellow medium. Mr. A. Brun's method is more complete
than my own, and both confirms and establishes it.
Lastly, Mr. Deby, Mr. Cox, Mr. Miiller and myself have endeavoured to
elucidate the structure of valves by examining the fragments of broken
valves. The study of these fragments which was pursued with objectives of
the greatest perfection (among which may be specially mentioned the
J
i-ioth Apochromatic of i*6o N.A. of Zeiss mounted in the most
( ) )
refractive media, such as monobromide of naphthalin, iodide of methylene
saturated with sulphur, the medium 2-4, etc., the injection of coloured liquids
as well as the examination of numerous preparations and photographs,
for which I am indebted to the kindness of Mr. Haughton Gill and
lastly the careful examination of an admirable preparation by Flogel,
(
I See ) Dr. H . Van Heurck. La nouvelle combinaison opticjue de M. Zeiss et la structure de la
valve des diatom^es, Anvers, 1890, in 8vo, with plates.