SOURCES OF MARINE FOOD. 
367 
cell is 1 millimeter deep, to 1,000 square millimeter surface, eaclr field of the eye-piece 
micrometer estimates 1 cubic millimeter out of the total 1,000 cubic millimeters 
inclosed in the Rafter cell; that is to say, as one counts the number of organisms in 
any one field of the microscope (a two-tliirds inch objective being used) it represents 
a one-thousandth part of the 1 cubic centimeter of filtrate under consideration.* 
The practice was observed of carefully studying over the entire field and then of 
tabulating the organisms from ten representative fields of the instrument, and these 
are the numbers given in the foregoing tables except the copepods, which were so few 
relatively and so large that it was my practice to count them with a lower power 
objective in the whole one cubic centimeter of filtrate under study. They are in 
numbers, therefore, one hundred times multiplied as compared with the other material 
tabulated. Two cells were always used in order to get a more complete representation 
of the 15 cubic centimeters of filtrate, and 20 or even 25 squares were counted from 
each 1 cubic centimeter in the Rafter cell until it was found that those averages did 
not very materially differ from the estimates based upon 10 squares, which were 
therefore finally taken as the basis of tabulation. 
The actual quantity of organisms per liter of littoral ocean water as computed by 
these data must therefore be obtained by multiplying each of the factors heretofore used 
in the tables ( except the copepods , which must be multiplied only by 3) first by 100 and 
then by 3. This gives the actual numbers of organisms in the normal ocean water as 
something truly wonderful, and I shall hope to substantiate or correct these estimates 
in the future, but for the present am convinced of the reliability of the comparative 
results, both because the material was handled so systematically in the same Avays 
throughout and because the end results compare so regularly. 
In order also to make these estimates as representative as possible those 
organisms were selected, as has been before stated, Avhich were most numerous and of 
constant occurrence in the material as it came under the microscope, and Avhich were 
provided Avith such skeletal elements as would resist dissolution in the process of 
filtration and preservation. And it is for this reason that although continual records 
Avere made of various other genera of infusoria, no general conclusions Avere based upon 
them, since they die so easily and wholly disintegrate. So, also, other diatoms of less 
usual occurrence were systematically recorded, though not given a place in the tables 
presented. All these plankton analyses, moreover, were made at the biological 
laboratory of Williams College during a busy term of teaching, at intervals, and 
the facts merely recorded and put on file, and it was only as such a study of the 
material was completed that the platting and tabulation were done, and -then for the 
first time were the relations of the organisms seen. 
These facts, and especially the harmony of surface, mid-depth, and bottom observa- 
tions at each point acting as checks upon each other, give evidence as to the truth of 
the distribution shown. The truth is all too partial, no doubt, even for the organisms 
cited, to say nothing of the Avhole series of animal organisms which were associated 
with them in their natural environment. But I am convinced of the value of numerical 
* This quantitative apparatus was first provided for me at the laboratory of the Boston Water- 
W orks in the autumn of 1889, where it is still in use ; this particular pattern of cell and the micrometer 
were designed, I believe, by Mr. G. W. Rafter, of Rochester, N. Y. It consists of a rectangular metal 
rim mounted upon an ordinary microscope slide, to be covered with a long coverslip. The inclosed 
contents then measure uniformly 1 mm. in depth. 20 mm. in width, 50 mm. in length, i. e., 1 cubic 
centimeter. 
