NOTES AND MEMOEANDA. 
221 
support of Delesse, the author points out the existence in the rocks 
in question of broken crystals, which have been recemented by the 
surrounding clay-slate substance. The author is thus led to incline 
towards the third theory, in favour of which some striking facts, 
drawn from the microscopical structure of the rocks, have already 
been adduced by Zirkel. He admits, however, that later metamorphic 
actions are not to be excluded in seeking to account for the origin of 
the crystalline constituents of clay-slates, and points out that four 
distinct stages must be considered in the series of changes by which 
the rocks in question have acquired their present character : — 1st, the 
deposition of the mud ; 2nd, the formation of minerals during the 
plastic state ; 3rd, the separation of materials during solidification ; 
and 4th, the action of metamorphic processes. 
Preparations of Infusoria, &c. — Professors Cohn (of Breslau) and 
Stein (of Prague) commend a new process, discovered by Herr 
Duncker of the “ Microscopic Institute ” of Berlin, “ after a series of 
long-continued and laborious researches, for preserving with the 
greatest possible truth to nature even the most delicate aquatic 
microscopic organisms, including many whose production has been 
hitherto considered an impossibility : ” these include Confervse, 
Desmidise, and Diatoms (with the organic parts preserved in their 
integrity), Ehizopods (Amoebae, &c.), Flagellata (Volvox, Crypto- 
monads, Astasiae), Cilio-Flagellata, Ciliata, Rotifers, Entomostraca 
(Cyclops, and particularly Daphnia, with the utmost possible preserva- 
tion of the soft parts), Tardigradia, and Larvae. It is suggested that 
such preparations will facilitate the determination of the microscopic 
fauna and flora of particular localities. 
The Binocular with High Pozoers.— Suggestions have recently been 
revived in American and English journals that Wenham’s ordinary 
binocular prism can be effectively used with the highest powers, 
inasmuch as by special modes of illumination — either by duplicating 
the light or directing it from two positions at a greater augle, as with 
a second mirror, the Paraboloid, or Powell and Lealand’s condenser — 
the two fields may be obtained free from any defect of light at the 
outer margin, both with a an A, or even a object-glass. In regard 
to this, Mr. Wenham writes that “his own opinion of the binocular 
microscope, in any form where the result is obtained from half the 
object-glass, is that in powers higher than one-fifth it ceases to be of 
much utility, as the combined images cannot compensate for the loss of 
definition in each eye. In the form of prism by means of which the full 
aperture is obtained in each eye with equal illumination in both tubes, 
the effect is much superior, and in lengthened observations with 
either high or low powers it affords great relief to be enabled to 
use both eyes continuously without regard to any special mode of 
illumination.” 
The Germ Theory of Disease. — The number of the Linnean So- 
ciety’s Journal issued on 23rd May,* contains the concluding portion 
of Dr. Bastian’s now familiar experiments with urine and potash, and 
* Vol. xiv., No. 74. 
