40 ROCHESTER ACADEMY OF SCIENCE. | March 10, 
considerable 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 easily and with as much 
certainty as in the middle. 
The placing of the cover glass is easily accomplished, although 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 may be noted that cleanliness is quite essential 
in all these operations, and the hints given by McDonald in his Water 
Analysis fully 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 considerable 
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 difficulty 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 cover 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. 
In the foregoing I have mentioned Prof. Sedgwick as the author o! 
the original method, and have spoken of it as essentially his. It is due, 
however, to other gentlemen to say that while Professor Sedgwick’s 
work in the way of making the method of practical value does 
undoubtedly entitle him to the honor of having it bear his name, never- 
theless, like all useful advances, it is the work of more than one person. 
