172 _ Transactions of the Boyal Microsco]pical Society. 
Dr. Kocli has never seen the flagella in B. termo, but he has 
made no effort to do so, because, as he tells us, he used low-angled 
glasses which are incompetent to the demonstration ; and has made 
no special provision for illumination, without which it is utterly 
impossible to see this fine organic fibre. Dr. Koch too, by using a 
method in which drying, staining, and mounting are involved, is, I 
am inclined to think, making the demonstration more difficult. He, 
however, without having made any effort to discover it, has no 
doubt that the discovery of it is a demonstration in the proper 
sense, although extremely difficult to make ; and he believes that 
the entire group of motile Bacteria are endowed with flagella. 
There is not the slightest doubt that this inference is correct. 
In all extremely delicate work with high^power lenses, the first 
difficulty is the greatest. If once an object has been seen, however 
difficult, it is immensely easier to see it agaiu. On the other hand, 
I have learned from experience that there is as great a diversity in 
different individuals in the sensitiveness of the retina, as there is 
in sensitiveness of the olfactory or auditory nerves. It is impossible 
to enable some persons to see objects beyond a certain limit of 
minuteness; as it is to enable others to detect certain scents, or 
hear notes pitched higher or lower than a given point. This is 
illustrated by the telescope as well as by the microscope, and has 
application to the practised as well as the casual observer. It is 
therefore fortunate that the constantly accumulating refinements 
of photography will ultimately provide us with a film equal to — 
perhaps finer than — the most sensitive human retina in its powers 
to fix the minutiae of detail, in form and structure, revealed by our 
highest powers and best lenses. But even then the efficiency of 
the results will he influenced by the delicacy of the retina, and the 
perfection of the eye of the manipulator. 
In the matter of the delicate flagella of B. termo the great 
difficulty had been overcome ; and for some time subsequently 
there was considerable fascination, in spite of the great difficulty, 
in repeating again and again the observation. At first we could 
only get the result with the “ supplementary stage ” illumination 
referred to in our joint paper. Since, however, I have succeeded 
with Powell and Lealand’s sub-stage condenser, and with Wenham’s 
reflex illuminator, using with this last apparatus, glycerine, letween 
the prism and the under side of the glass slip. 
The secret of success, skilful manipulation and the right kind 
of lens being assumed, is the manner in which the Bacteria are 
prepared. They should be taken from an old and thick maceration ; 
not a recent infusion with a thin fluid ; and then should he very 
gradually accustomed to thinner and thinner fluid, until, by two or 
three days’ habituation in Cohn’s fluid, for a few hours they will 
live in water. And it is in this that they should be examined; 
