1892.] MICKOSCOPICAL JOUKNAL. 61 



sort of mental standartl as to the unit of area covered by one mass of the 

 amorphous matter. Mr. Geo. C. Whipple, who has assisted me in some 

 experimental work for the Boston Water Works, has suggested that 

 this unit be made definite for all persons by taking it a fixed number 

 of square microns, and for this purpose 30 microns seems to be the 

 desirable unit. By careful comparison with a stage micrometer for a 

 few times, this unit can be firmly fixed in mind and an estimate of the 

 amount of amorphous matter made with considerable precision. 



The advantage of a cell of such depth as to just contain the 

 quantity taken for examination is illustrated by figure 3, which repre- 

 sents the open cell and shows the meniscus form taken by the liquid, by 

 reason of capillary attraction at the sides. This curvature is so con- 

 siderable as to render a count in the squares near the edges of the 

 cell impracticable, for optical reasons, which every user of the micro- 

 scope will readily understand. With the covered cell, on the contrary, 

 the count may be made up to the sides as easil}' and with as much 

 certainty as in the middle. 



The placing of the cover-glass is easily accomplished, altliough the 

 careful observance of certain details are essential to uniform success. 

 Thus the cover-glass should be perfectly clean, and just before placing 

 should be moistened. The operation of putting it to place consists in 

 laying one end, held in a horizontal position, in contact with the ground 

 upper surface of the metallic portion of the cell, and, while keeping 

 it in close contact at all points, gradually sliding it forward until the 

 whole cell is covered. In this connection it mjiy be noted that clean- 

 liness is quite essential in all these operations, and the hints given by 

 McDonald cover the case. 



In the original cell, as designed by Prof. Sedgwick, the division into 

 squares for the purpose of obtaining the relation of organisms to area 

 was arrived at by ruling square millimetres upon the upper surface of 

 the glass slide on which the cell is based. This, however, gives a 

 unit square only for the bottom of the cell, and for all organisms at the 

 top of the liquid no unit of area is obtained, inasmuch as the consid- 

 erable change of focus required in order to see them at all renders it 

 impossible to distinguish the ruled lines and such floating objects at 

 the same time. With the' eye-piece micrometer, however, this difii- 

 culty is removed, and the unit square is clearly in the field of vision 

 without reference to the plane in the cell upon which the objective 

 is focussed. 



The working objective for these counts may be either a two-thirds or 

 one-half inch, and for identification of minute unknown forms a one- 

 fourth or one-fifth water immersion capable of working through a thick 

 covei'-glass and cell one millimetre in depth would be useful. I have, 

 however, no experience with a high power objective of this character, 

 and can only cite the opinion of our Rochester opticians that such an 

 objective of satisfactory correction and definition can be made. 



Mr. A. L. Kean first used a small cell, made to contain i cubic milli- 

 metre, early in the winter of 18S8-S9, and attempted by the use of 

 such a cell to arrive at a quantitive determination of the number ot 

 organisms present in a given sample. Such a cell was found to be 

 altogether too small to furnish other than uncertain results, although it 

 probably suggested the larger cell which has become of great value. 



