108 Transactions of the Boyal Microscopical Society, 
the same object ; but by bringing it into delicate focus it presented the 
aspect seen at 1 a (Fig. 6), which really represents the same object 
at the same time only with the other end in focus, while the end 
marked d corresponding to 2 & of Fig. 6 was in its turn slightly 
out of focus and the flagellum lost to view. This observation, made 
together, was as satisfactory as could be desired ; and it was thus 
demonstrated that there was a flagellum at both ends of the ordinary 
B. termo. 
It will of course be understood that it is by no means an easy 
matter to secure the demonstration ; and whenever we repeat it, it 
must always be with nearly the same amount of trouble and 
patience, although we can now with the ordinary condenser detect 
the vortical action both in front and (occasionally) behind the termo 
as we never did before. But the demonstration of the ultimate 
structure of a fixed object — as for instance Amjphipleura pelliir- 
cida — must be looked upon. as a matter of great ease in comparison; 
and that for many reasons, the foremost being the motion and 
minuteness of the object, the swift play of the flagella, their simi- 
larity in optical properties to the fluid in which the bacteria live, 
the difficulty of retaining them in focus, and of getting them in 
such a position in relation to the light as to make demonstration 
possible. Of course all this would be removed if dead bacteria 
would answer, but they very rarely (indeed only once) have done 
so with us. The flagellum needs to be in slow motion to properly 
show itself ; for even with the more delicate and minute monads it 
is a difficult thing to show the flagella in dead forms, since in the 
majority of cases they appear to be attracted round the body of the 
creature. 
