68 
NATURE 
[May 22, 1873 
absorbed and may afterwards be communicated to other 
surfaces. 
The importance of this discovery can scarcely be over- 
rated, and there is no doubt but that it will work an era 
in the matter of carbon printing. We need secure but 
one single photograph printed in the sun in order to 
obtain a large number of copies, all of which shall be as 
delicate and vigorous as if they had been printed by sun- 
light. A sheet of gelatine sensitised with bichromate of 
potash is put under a negative and printed ; it is with- 
drawn from the printing fram2 and immersed in a weak 
solution of bichromate of potash which swells up those 
portions of the surface that have ret been attacked by 
light, and thus produces a picture in relief. The sheet 
of gelatine is then put into a press and impressions from 
it taken on sensitive carbon tissue, the block being 
moistened from time to time with bichromate solution. 
The copies thus produced upon the tissue are not fully 
printed and cannot be developed at once; they are 
simply incipient, or nascent, pictures, it must be men- 
tioned, and they require preservation in the dark for 
some hours to allow the action of the light to continue, 
exactly in the same way as if the carbon tissue had been 
exposed to sun-light for a few minutes. When the prints 
have been kept sufficiently they are developed in warm 
water, and fine vigorous copies are the result. Naturally 
enough if the tissue is kept too long after, the mordant 
action of the light continues rendering the film insoluble, 
and then the development of the image in warm water 
obviously becomes impossible. 
Another application of the same principle has been 
made by M. Marion, in which carbon printing is assimi- 
lated to silver printing, to such a degree, that those 
accustomed to the ordinary method of printing photo- 
graphs on albumenised paper, would find no difficulty in 
adopting it. H. BADEN PRITCHARD 
CN THE METHOD OF COLLECTING AND 
PRESERVING ENTOMOSTRACA AND 
OTHER MICROZOA 
ONSIDERING the varied interest which attaches to 
the Entomostraca, it has long seemed to me that they 
attract a remarkably small share of attention from micro- 
scopists. In the case of so widely distributed and 
numerous a group, this cannot arise from any real diffi- 
culty in procuring materials for study ; but I believe it 
does arise in great measure from a want of information 
as to the best means of capturing and preserving speci- 
mens. I propose, therefore, briefly to point out some of 
the methods which in my own hands have best answered 
these ends. 
Classification.—The Entomostraca constitute, as all 
microscopists know, a division of the class Crustacea, and 
for the purposes of the present paper we may with suffi- 
cient approach to accuracy consider them as forming four 
groups—C/adocera, of which the common Daphnia, or 
water-flea, is the type ; Ostracoda, typified by the little 
hard-shelled, bivalve, mollusc-like Cyfris ; Copepoda, re- 
presented by the well-known Cyclops; and the parasitic 
species, Peczlopoda, commonly known under the name 
“ fish-lice.” 
Respecting the last-named group, I shall have nothing 
to say here ; the mere knowledge of their mode of life 
indicates the method of capture. 
Habitat.—All collections of still-water, large and small, 
from the mere road-side pool to the mountain lake and 
the ocean, support, with scarcely an exception, their 
quota of entomostracan inhabitants; nor is purity an 
essential condition of their existence, for sometimes they 
are found in great numbers when one would think the 
foulness of the medium too much for animal existence of 
so high a grade. Doubtless, however, a moderate purity 
of water is necessary to the presence of any great variety 
of species ; a luxuriant aquatic vegetation is also very 
favourable to the growth of most Entomostraca, affording 
them probably not only tood, but shelter. For this reason 
the weedy margins of lakes are as a rule much more pro- 
lific than the clear central portions, where, indeed, but 
little microscopic life usually exists. Rapidly flowing 
water is of course unfavourable to the existence of these 
organisms, but the sea, both betwéen tide-marks and in 
the open, abounds with them. Ostracoda, except the 
fresh-water Cyprides, live for the most part on the bottom, 
and are therefore to be obtained chiefly by dredging. The 
brackish water of salt-marshes and estuaries supports its 
own peculiar species, some of which often occur in pro- 
digious numbers ; and even the highly saline waters of 
brine springs and salt lakes have been found to contain 
Entomostraca. 
Methods of Collecting 
1. Freshwater—An ordinary “ring-net,” made of 
“hard muslin,” or “ crinoline,” from six to twelve inches 
in diameter, and fitted to the end of a walking-stick, will 
be found the most convenient apparatus for the capture 
of such swimming species as haunt the weedy margins of 
ponds and lakes. For such shallows as are matted with 
a growth of Zi¢tored/a, Lobelia, or other dwarf ground-plants 
a “horse-shoe” net, with a frame made after the fashion 
of a Dutch hoe, is very serviceable; while in working from 
a boat in the centre of a lake the ordinary ring-net on a 
stick will be quite sufficient. In this way the net will, 
after working for a few minutes, usually be partially 
filled with fragments of weed and other débris, amongst 
which there will also be found a fair sample of the Mi- 
crozoa inhabiting the locality. The coarsest fragments, 
such as stems of rushes and portions of water weeds, may 
conveniently be picked out with the fingers, and thrown 
away, while the rest of the contents of the net must be 
transferred to a bottle of clear water, an eight-ounce being 
a convenient size for the purpose. The Microzoa may 
then be readily separated by filtering into another bottle 
through a net of sufficiently wide mesh to allow of their 
passage through it : “ mosquito-netting” I have found to 
answer well for this purpose. Having thus obtained our 
Entomostraca in a condition tolerably free from admixture 
with extraneous matter, they may easily be collected in a 
patch on the centre of a piece of fine muslin by passing 
the whole through a piece of that material, arranged over 
a funnel. They should then be transferred at once (if it 
be not wished to keep them alive) to a small phial of 
some preservative fluid. This may be effected easily by 
a penknife, but a very convenient instrument for the pur- 
pose is an ordinary quill toothpick. This process, which 
appears somewhat cumbrous in writing, is in reality very 
easily performed, but it may be still further simplified, ac- 
cording to the fancy of the collector, by fitting an outside 
funnel with a muslin net, and having a small inner one 
of perforated zinc, so as to do all the filtering at one 
operation, The collecting net may also be protected 
from the entrance of very coarse rubbish by a light, move- 
able wire grating. The species obained by these means 
will often include numerous representatives of all three 
orders, Cladocera, Ostracoda, and Copepoda. For the 
capture of such Ostracoda as haunt the bottom in parts 
too deep to be reached by a walking-stick, a small hand- 
dredge is required : this will be more particularly noticed 
in the marine section. 
2. The Sea.—The free-swimming species, the great 
majority of which belong to the order Copepoda, may be 
most conveniently captured by the walking-stick net held 
over the side of a row-boat in gentle motion. Care should 
be taken that the lower end of the net is as wide or wider 
than its mouth, and that the material, while close enough 
to retain the Entomostraca, is yet open enough to allow a 
free current of water through it: if those points be not 
attended to the result will be a back-wash, carrying back 
out of the net much which should have been retained, 
