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. = n sin 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. 1 



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 



1 Abbe found that the brightness of the image varies as the square of the numer- 

 ical aperture. 



2 An Abbe condenser of the commonly purchased form has as its maximum a 

 N.A. of i. 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. 



