18 



directions of Calkins*, Jackson+, 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. For 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 & Schiill. The water from which the sample for 

 filtration is taken has been obtained by means of the pump. It 

 Avas 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 light 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 libers of its surface. To obviate this ditticulty 

 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. (J. N.— The Microscopical Examinadon of Water. Hep. Afass. State Board 

 of Health, 1891. pp. 39G-4L>1. i folding tables. 



•|Jackso7i, D. D.—.\n Improvement in the Sedgwick-Rafter Method for the ilioro- 

 scopical Examination of Water. Tecli. Quart, ^'ol. ix., pp. 271—274. 1806. 



JWhipple, G. C— Experience with the Sedgwick-Kafter Method at tlie Biological 

 Laboratoi J- of the Boston Water Works. Ibid., pp. ■-'7."5--'79. 1891). 



