8 
ELEMENTARY CHEMICAL MICROSCOPY 
objective to make clear objects or structures in more than one 
plane. This is known as its penetrating power. The pene¬ 
trating power of an objective has been shown to be inversely 
proportional to the numerical aperture and to vary as the square 
of the equivalent focus. 
Leaving out of consideration the numerical aperture, it is 
found that the resolving power of an objective is inversely pro¬ 
portional to the wave-length of light. By employing light rays 
of very short wave-lengths we may thus obtain exceptional 
resolution. 
In the consideration of numerical aperture it is usually assumed 
that the illuminating cone of light completely fills the aperture of 
the objective. Nelson ^ has shown that in practice with the older 
types of objective we can rarely count upon more than three- 
fourths of the available numerical aperture. Modern objec¬ 
tives perform somewhat better. 
In comparing objectives as to their ability to render struc¬ 
tures clear and distinct it is usual to do so by computing the 
number of ruled lines to the inch or millimeter each one will 
make clearly visible (resolve). Since, as pointed out, we can¬ 
not obtain the theoretical resolving power in practice a correc¬ 
tion coefficient must be introduced into our formula. Nelson 
assigns to this coefficient the value 1.3. The practical working 
formulas then become 
Available resolving power = - 
1.3 X 
Available illuminating power = 
For white light a mean value may be assumed to be X = 5607 
(= 0.5607 and for blue light X = 4861 (= 0.4861 /x). 
Advantage has been taken of the increased resolving power 
^ J. Roy. Micro. Soc., 1893, 
2 J. Roy. Micro. Soc., 1906. 521. 
