OF THE DESMIDIACEiE AND SIMILAR MINUTE ALGAL 59 
first great need now is to see that not a trace of glycerine 
remains outside the ring enclosing the object. Should there 
be any, it must be removed thoroughly before any further 
steps are taken. If satisfied that the glass is quite clean, a 
line of asphalt must be drawn with a soft hair pencil 
round the whole edge of the cover, so as to include it and 
its place of contact with the slide. It must be done slowly, 
and the slide watched, to see that no bubbles of air, or any 
foreign bodies, are enclosed within the circle. This layer 
of cement having dried, a second, third, and even fourth 
may be added, until the preparation may be reasonably 
supposed to be air-tight. This may be most readily tested 
by holding the slide between the eye and the light, and 
seeing whether any part covered by the cement is trans- 
parent, especially the corners, if the covering glass is 
square. Any semi-transparent spot, or line, must be re- 
painted with cement. The cement, it may be added, 
should be very thin when in use. It is then laid on much 
more easily, and tends to make the preparation more truly 
air-tight. A thick cement dries badly, and is apt to crack 
and chip off, especially during changes of temperature. 
The preparation may now be considered complete. The 
name and other particulars will of course be added, and it 
may, if desired, be covered with ornamental paper.. One 
most necessary point must not be forgotten, that is, under 
all circumstances, to lay the slide in a horizontal position. 
If set upright, the contents flow to one side, get heaped 
together, and eventually the fluid is almost sure, by its 
weight, to force its way through the cement, and cause a 
fatal leakage. 
[Since the above was in type, I have had the opportunity 
of examining slides of Desmidiacete, prepared in Dresden 
after Ilerr Hantzsch’s method. Nothing can exceed the 
beauty of these preparations ; the Jorm of the plant and 
the colouring of the endochrome having undergone no 
change whatever. — Ed.] 
As to the characters most worthy of observation, nearly 
