832 
variety of situations made at intervals 
throughout more than a year indicate that 
the silk net retains from one-half to one 
forty-fifth of the total solid contents of the 
water, the greater losses occurring from 
waters containing Zrachelomonas, Chlamydo- 
monas, Huglena, Melosira and other minute 
formsin abundance. The relative amount of 
silt is, however, much greater in the filter- 
paper catches than it is in those made with 
the silk, so that the actual volume of plank- 
ton lost is less than the above figures indi- 
cate. The amount escaping through the silk 
bears no constant relation to the amount re- 
tained. Under these conditions the volumet- 
ric determination of the plankton by the 
use of the silk net as a test of the produc- 
tiveness of water is not only incomplete but 
may be misleading. 
For the examination of the plankton by 
the statistical method the silk affords a 
satisfactory basis only for the larger forms, 
such as the Entomostraca and the larger 
Rotifera and Protozoa. For the smaller and 
often very abundant planktonts, such as 
Melosira, Peridinium, Dinobryon, Raphidium, 
Scenedesmus Huglena, Trachelomonas and Chla- 
mydomonas, the Hensen method is wholly in- 
adequate. For example, from water in which 
these smaller forms were not extremely 
abundant the silk retained organisms to 
the number of 248,200 per cubic meter, 
while the catch of the Berkefeld filter indi- 
cated the presence of 767,556,000 planktonts 
in the same amountof water. Many of the 
organisms listed in the counting tables of 
Apstein may in reality escape in large num- 
bers through the silk. Thus, of Codonella as 
many as twenty-one individuals may escape 
to one retained, The Hensen method must 
be supplemented by a more accurate system 
of collection if a complete census of the 
water world is to be taken. 
From the cecological point of view the 
plankton lost by leakage through the silk 
s of prime importance, for it is composed 
SCIENCE. 
[N. S. Vou. VI. No. 153. 
very largely of minute alge, which consti- 
tute a fundamental link in the eycle of 
aquatic life. Any attempt to unravel the 
complex interrelation of the constituents of 
the plankton or to correlate its ever-pro- 
gressing changes with the factors of its en- 
vironment must be based upon reliable data. 
Biological theory and aquaculture alike de- 
mand improvement in the plankton method. 
The errors enumerated above are doubt- 
less exaggerated by the situation with which 
we deal—waters rich in plankton and more 
or less turbid with silt. The tests, however, 
cover a considerable seasonal and local 
range of quantity and constituency, and 
have been made in both clear and turbid 
waters. The plankton, moreover, is com- 
posed very largely of the same genera as 
those found in the lakes in which Apstein 
and Zacharias have carried on their inves- 
tigations, and over 50% of the species are 
identical. The desirability of experiments 
in other waters is at least suggested. 
C. A. Kororp. 
ILLINOIS BIOLOGICAL STATION. 
SOUTHERN STAR-CLUSTERS. 
Tue last of the great contributions of 
Benjamin Apthorp Gould to astronomy is 
contained in the large volume recently pub- 
lished under the title Cordoba Photographs : 
photographie observations of star-clusters from 
impressions made at the Argentine National 
Observatory. This work gives the measure- 
ments of the relative positions of nine 
thousand stars included in thirty-five 
clusters of the southern heavens and in the 
Pleiades and Preesepe. 
In addition to the other large enterprises 
which constituted the regular work of the 
Argentine Observatory, over twelve hun- 
dred plates of southern clusters were 
secured (no important one being omitted) 
in the decade beginning in 1872, of which 
281 have been measured and 177 are now 
computed. Inasmuch as the dry plate pro- 
