6 
ELEMENTARY CHEMICAL MICROSCOPY 
the preparation evidently pass from a denser medium (object) 
to a rarer medium (air), and following the law of refraction are 
bent away from the perpendicular. Hence part of these light 
rays are lost, since they are bent so far that they cannot enter 
the small front lens of the objective. To prevent this loss and 
secure a brilliant image it is necessary, according to the formula 
N.A. = wsin a, to increase the value of n. Therefore, to obtain 
very high powers, the substitution of some liquid, for air {n = i) 
between the objective and the preparation becomes imperative 
in order that the image may be bright and distinct.^ 
Objectives permitting the use of a liquid in this manner are 
known as immersion objectives. When water is employed {n = 
1.33) they are called water immersion, and when an oily liquid, 
oil immersion. Usually the oil consists of slightly thickened 
oil of cedar wood {n = 1.52), and since the refractive index of 
glass object slides, cover-glasses, and the lower or field lenses of 
the objectives is approximately 1.52 also, such objectives are more 
commonly designated homogenous immersion objectives. Alpha 
monobrom naphthalene is also sometimes used as an immersion 
fluid {n = 1.66) and gives us the highest numerical aperture 
obtainable.2 Since oil-immersion objectives have the highest 
numerical apertures they therefore yield the brightest and the 
clearest images, and represent the highest development in the 
art of microscopic objective manufacture. 
In the case of immersion objectives the working distances are 
often greater than the equivalent foci. 
Variable Objectives are so constructed that the distances 
between two sets of component lenses may be changed by means 
of a graduated collar, permitting a wide range in the magnifying 
power of the objective. A single objective is thus made to do 
the same work as a number of objectives of fixed system. For 
^ Abbe found that the brightness of the image varies as the square of the numer¬ 
ical aperture. 
* An Abbe condenser of the commonly purchased form has as its maximum a 
N.A. of 1.20; while the three lens condensers of the highest type will transmit 
rays only up to a numerical aperture of 1.40. Unless therefore a special achromatic 
condenser is available, it is manifestly useless to employ alpha monobrom naphtha¬ 
lene immersion objectives, since only a part of the full aperture will be available. 
