1901] MICROSCOPICAL JOURNAL. 167 
ful for the purpose. A hand-fed lamp must be employed. 
When this is properly adjusted and the condensing lens 
is in position, a luminous disc will be seen upon the leaves 
of the partially closed Iris diaphragm of the substage con- 
denser. During an exposure it will only be necessary to 
maintain this disc in a fixed position by turning the mill- 
ed head of the lamp very gently as required, and the 
light may be kept perfectly central for any length of time. 
It is presumed that a horizontal camera would be used. 
STAINING FLAGELLA.—The preparation of Bacteria so 
as to exhibit flagella has always seemed to be unsatisfac- 
tory and difficult. Very few workers are really success- 
ful and none have produced permanent mounts. An in- 
teresting note occurs in the Thompson Yates Laboratories 
Report, by Dr. MacConkey, which deserves consideration. 
It has been considered essential when staining such 
preparations to use a mordant, presumably to fix the dye 
in the substance of the flagellum. It is suggested that 
the rendering visible of the flagella in consequence of the - 
use of the mordant is not because of the effect which it 
has hitherto been credited with producing, so much as by 
causing the flagellum to swell and become thicker. The 
flagella are of exquisite tenuity, so much so, that when | 
stained, the dyes do not seem to render them visible to 
the same extent as when a so-called mordant is used. The 
suggestion put forward is confirmed by the statement that 
‘ the flagella appear to be thicker than they are supposed 
to be actually, and the organisms themselves are larger 
after the use of a mordant than when stained in the or- 
dinary way. 
There are dyes which have the effect of staining the 
flagella deeply and producing a thickening, but it is ob- 
served that, as these colors fade, the flagella become in- 
creasingly fine until at last they are no longer visible, 
This is a subject in which, to the ordinary microscopist, 
few opportunities are afforded of making experiments % 
