130 A MANUAL OF BACTERIOLOGY 



light must be diminished by closing the diaphragm, lowering 

 the condenser, etc. (p. 132), and artificial light is generally 

 preferable to daylight. The central parts of the drop 

 only should be examined, not the margin. 



Instead of hollow slides, various devices may be em- 

 ployed to form the cell. Metal, glass, or vulcanite rings, 

 or rings cut out of thin sheet lead, tin-foil, cardboard, or 

 two or three thicknesses of paper or filter-paper may be 

 cemented on to slides with vaseline, Hollis's glue, gold 

 size, or Canada balsam, or a thick ring of vaseline, or 

 paraffin, or plasticine may be used. 



The only certain method for ascertaining whether an 

 organism is motile or not often an important clue to its 

 identification is by the use of hanging-drops. Actively 

 motile organisms may frequently assume a non-motile 

 resting stage, although still alive, and various factors 

 may bring about this condition, such as old age, exhaustion 

 of nutriment, excessive heat or cold, electric shocks, and 

 the like. The absence of movement of an organism in a 

 specimen prepared from an ordinary culture, particularly 

 if more than a day or two old, does not necessarily prove 

 that it is non-motile. A hanging-drop should be prepared 

 with a nutrient medium (the best, perhaps, is glucose 

 broth) and placed under conditions of temperature, etc., 

 favourable to the growth of the organism, and examined 

 after an interval of an hour or so, or better still at intervals 

 of half an hour for three or four hours. In this time the 

 old cells will revivify, and new ones will have been pro- 

 duced, and if the organism be a motile one, more or less 

 active movement of some of the cells is almost sure to be 

 observed. It is necessary to beware of two fallacies in 

 connection with motility not to mistake for it the so-called 

 Brownian movement, which is a vibratory one back- 

 wards and forwards about one point, and common to 

 all fine particles suspended in a fluid ; and not to be 



