350 TECHNOLOGY OF DIATOMS. 



them jump in a manner that is not pleasant to the operator. 

 Second, the bristle has touched the Diatom too much on one side, 

 and according to its shape has made it take a half-turn, accom- 

 panied by a jump. 



To avoid these, it is necessary, in the first case, to cause the 

 valve to slide very smoothly before the bristle, sometimes breath- 

 ing softly on the plate and not lifting the valve till it is free ; and 

 in the second case, which is the more frequent, only touching the 

 Diatom on the centre, which is what you always ought to do. 



The Diatoms stick to the Bristle. 



I have said, in speaking of the choice of a bristle, that it is 

 needful that it should be taken either from a /iving a.n[ma\ or from 

 one recently killed. Under these conditions, preserving its natu- 

 ral fatty condition, it remains supple and the objects adhere to it 

 more wilhngly. It will be enough to pass the bristle through the 

 lightly-closed lips to make it sufficiently moist to hold the Dia- 

 toms, and at the same time to remove any particles of dust that 

 may be adhering to it. When the valves do not easily leave the 

 bristle, it is that a little saliva remains on the end of it, or perhaps 

 that the extremity presents some accidental roughness, in which 

 case it becomes necessary to change it ; lastly, this tiresome adhe- 

 rence may equally proceed from a cause that I attribute to an 

 electric current — a curious fact, but one well known to those who 

 have selected Diatoms. Mr. Kinker, of Amsterdam, has told me 

 that when the north wind blew rather strongly, the effect was very 

 sensible. I have observed the same many times. This is not 

 always the sole cause ; but each time that an effect of this kind 

 has been produced, I have laid aside my selection for a more 

 propitious moment. 



When a Diatom has been chosen, it must be transferred to a 

 slip prepared to receive it, and there fixed. For this it is needful 

 to use two microscopes, the second serving to centre the slip 

 carrying the prepared cover-glass ; but then there is some chance 

 of losing the object en roufe, especially if it be heavy. It is 

 therefore better to place the two slips side by side on the stage of 

 the microscope which is used for the selection, and then advancing 



