18 
directions of Calkins*, Jacksont, and Whipple}. The loss by 
leakage from this filter was so great that we abandoned it and 
have been using filter paper as a supplementary method of 
collection since September, 1896, in all regular plankton 
examinations. Tor a short time ordinary filter paper was used, 
but owing to the looseness of its texture and consequent entangle- 
ment of the plankton and shedding of lint we rejected this 
paper and have since used the ‘‘ hardened filter paper,” No. 575, 
Schleicher & Schtll. The water from which the sample for 
filtration is taken has been obtained by means of the pump. It 
was often necessary to take a much larger amount than was 
used for filtration in order to secure a vertical collection. To 
obviate this and also to secure greater accuracy in the collection 
of a vertical sample, a vertical water-trap was devised, which con- 
sists of a ight brass tube three inches in diameter and eight 
feet long, at whose lower end is a sliding brass gate by which 
the bottom of the tube can be closed after it is lowered to the 
desired depth. | 
Although the filter-paper collections served to correct the 
loss by leakage in an important degree, the method was defec- 
tive in that a small portion of the catch, varying with the amount 
and character of the plankton, remained on the filter paper, 
entangled in the fibers of its surface. To obviate this difticulty 
and to secure, if possible, a method which would be effective 
and permit the handling of a large quantity of water, experi- 
ments were made with the centrifuge. The small machine 
described in the last report and adopted by us for use in the 
measurement of plankton collections was found to precipitate a 
large per cent. of the organisms present in the water, accord- 
ingly a larger machine was devised and built at the Mechanical 
Shops of the University for this purpose (see Plate X.) It 
consisted of a hollow cylinder axis of gun-metal with two return- 
ing arms, each bearing at the elbow a detachable receptacle 
which receives the solid matter precipitated from the water, 
which last is passed through the revolving axis and out to the 
*Calkins, G. N.—The Microscopical Examination of Water. Rep. Mass. State Board 
of Health, 1891, pp, 896-421. 2 folding tables. 
+Jackson, D. D.—An Improvement in the Sedgwick-Rafter Method for the Micro- 
scopical Examination of Water. Tech. Quart, Vol. ix., pp. 271—274. 1896. 
+Whipple, G. C.—Experience with the Sedgwick-Rafter Method at the Biological 
Laboratory of the Boston Water Works. Ibid., pp. 275-279. 1896. 
7 oo 
