OF THE DESMIDIACE^E AND SIMILAR MINUTE ALGM. 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 Desmidiaceae, prepared in Dresden 

 after Herr Hantzsch's method. Nothing can exceed the 

 beauty of these preparations ; the form of the plant and 

 the colouring of the endochrome having undergone no 

 change whatever. — Ed.] 



As to the characters most worthy of observation, nearly 



