74 MICROSCOPICAL TECHNIQUE. 



to spread, yet should flow readily and smoothly from the brush. 

 The depth of the cell should be such that a complete support 

 shall be provided for the cover-glass without causing it to bear 

 upon the object when cemented down, and yet should not be of 

 such a depth as to interpose an unnecessary stratum of glycerine 

 between the section and the cover-glass. Of more importance, 

 perhaps, than any other point is the direction regarding the age of 

 the cell. It is a common practice to ring a cell and use it while 

 fresh, the manipulator arguing that a more perfect union of cell- 

 wall and cover-glass is secured in this manner. Perhaps this is 

 true ; but it is at the expense of the slide's usefulness. The author 

 already quoted is authority for the statement that any ordinary 

 balsam cell will, in drying, shrink 30 per cent. Under these con- 

 ditions, and in view of the fact that glycerine is non-compressible, 

 something must give way when the cell contracts, and this is either 

 the cover-glass or the cell-wall. Whichever it is, the final result is 

 the destruction of the mount and loss of all the work involved in 

 its preparation. This leads us, then, to make the following state- 

 ment : — Never use a green cell. The older the cell the better, and, 

 at ordinary temperature, two weeks is the shortest space of time 

 in which a cell of medium depth will become seasoned. ... In 

 placing the section care should be exercised to have it exactly in 

 the centre of the cell. With the section thus situated, a drop of 

 glycerine is allowed to fall upon it from the dropping-bottle. Take 

 the clean cover-glass and place the left side in contact with the 

 drop of glycerine ; draw it over until supported on the left edge 

 of the cell-wall . . . and allow the cover-glass to fall gradually 

 by supporting the right edge with a needle. Having thus placed 

 the cover-glass and centered it, place a clip upon it. The super- 

 fluous glycerine thus forced out is washed away by means of a jet 

 of water from the wash-bottle, so directed as not to strike the 

 cover-glass. Some water does get under, but this does no harm, as 

 it supplies moisture which the glycerine otherwise would have by 

 ' creeping ' from the cell. When thoroicghly dried by means of 

 strips of bibulous paper, the slide is ready for the last step — 

 securing the union of cover-glass and cell-wall. This result 

 is best obtained by ringing once around the cover-glass, and 

 allowing this coat to dry before applying cement enough to 



