28 BACTERIOLOGICAL APPARATUS 
been standardized. Variations in the length of this draw tube should 
be made when cover glasses of a different thickness are used. 
Numerical Aperture. N.A.=7 sin u. 
n=the lowest refractive index that appears between the object 
and the front lens of the objective; 
u=half the angular aperture. 
The numerical aperture is important since it determines the resolv- 
ing power definition and illumination. It is especially important with 
reference to the resolving power. The resolving power is directly pro- 
portional to the numerical aperture. The higher the numerical aper- 
ture the greater the resolving power and the finer the detail. Bausch 
has pointed out the significance of the N.A. (numerical aperture) as 
follows: 
“Tf a very narrow central pencil is used for illumination, the finest 
detail that can be shown by a microscope, with high enough magni- 
fication, is equal to rae where \ is the wave length of the light used 
N. 
for illumination. The wider the pencil used for illumination, the greater 
the resolving power, until a maximum is reached, when the width of 
the pencil is sufficient to fill the whole aperture of the objective. In this 
case the resolving power is twice as great, the finest detail that the 
objective can show being now equal to 2 N.A. This same limit is 
reached when a narrow pencil of greatest possible obliquity is used. 
For example, the wave length of the brightest part of the spectrum 
may be assumed to equal 0.00053 mm. Consequently an objective of 
N.A. equal to 1.00 will resolve two lines separated by a distance of 
0.00053 
1.00 — 
0.00053 
21.00 
narrow oblique cone. 
“A 4 mm. 0.85 N.A. objective will resolve lines separated by dis- 
tance ranging between 0.00062 and 0.00031, dependent upon the aperture 
employed. For a 4 mm., 0.65 N.A. objective the limiting values are 
0.00081 and 0.000405. . 
“The N.A. can also be expressed by the equation. 
equal to 0.00053 with a narrow central illumination cone, and 
equal 0. 000265, with a cone filling the whole aperture, or with a 
N.A, == SHective aperture of back lens 
"Of 2Xequivalent focus 
“Two objectives of the same equivalent focal length (E.F.) and the 
same N.A. should show the same illuminated area in the back lens, 
