The Flagella of Bacterium termo. By W. II. Ballinger. 173 
for the comparative opacity of the natural fluid makes it impossible 
to see the flagella, and in the water there is obviously a greater 
contrast between it and the sarcode of the flagella, than there is in 
the thick decomposing fluid. And I have repeatedly observed that 
if the Bacteria are taken from a recent infusion which is little more 
in substance than water, — probably from some difierence in the 
density of the sarcode, — the flagella cannot be discovered. 
After repeatedly, and in different ways, having demonstrated 
the flagella, it struck me that much would be gained if it were 
possible to measure its diameter. To do this by a direct method 
was impossible with any instrument with which I am acquainted ; 
but it might, it appeared to me, be done with very approximate 
accuracy by an indirect method. To this end I made a series of 
investigations with Powell and Lealand s “ new formula ” ^ objec- 
tive, and with what appeared to me extremely interesting results. 
But subsequently they furnished me their yV and yV inch lenses 
on the same formula. And as immersion lenses their performance 
is remarkably beautiful. I also had a very fine immersion -yV by 
the same makers ; and at my request they made me a yV? fii’st 
they had ever made. This lens was made with a view to the special 
class of observations in which I have been engaged, and it is an 
extremely beautiful one. The angle is moderate, and the lens with 
the same front is both immersion and dry. Its definition when 
properly used is very crisp and clear, and its “ penetration,” con- 
sidering its magnifying power, is very considerable. 
My purpose was, having been furnished with these lenses, to 
make a series of indirect measurements with each of these lenses 
separately, and then compare the results and obtain an average. 
My method was as follows, viz. : — 
1. It was necessary, and comparatively easy, to measure accu- 
rately the absolute diameter of the Bacterium body, as, for example, 
in Fig. 10, Plate IX., to find the actual distance from x to y. 
2. Next it was needful to make a very careful camera lucida 
drawing of the body and a part of one of the flagella. The micro- 
scope being in an upright position the ordinary camera lucida 
cannot conveniently be used ; but an extremely useful instrument, 
made by M. Nachet, of Paris, to meet this emergency, answers 
admirably. It is, indeed, much easier to draw with than the 
ordinary camera lucida ; and in using it with high powers all that 
is required is, that the right hand employed in drawing be illumi- 
nated a little more intensely than the “ field,” when it will appear 
extremely “ ghostly,” but very sharp and clear, and the pencil 
point is admirably defined. 
A fine white surface is needed which will receive a mark 
without such rough edges as are made, with even very hard pencils, 
on the finest London or Bristol cardboard. That which I find to 
