ELEMENTARY CHEMICAL MICROSCOPY 



also, such objectives are more commonly designated homog- 

 enous immersion objectives. Alpha mono-brom naphthalene 

 is also sometimes used as an immersion fluid (n = 1.66) and 

 gives us the highest numerical aperture obtainable. 1 Since oil 

 immersion objectives have the highest numerical apertures they 

 therefore yield the brightest and the clearest images, and repre- 

 sent the highest development in the art of microscopic objective 

 manufacture. 



In the case of immersion objectives the working distances 

 are usually greater than the equivalent foci. 



Variable Objectives are so constructed that the distances be- 

 tween 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 low powers, the chemist 

 will find an objective of this sort an ex- 

 ceedingly great convenience. Fig. i shows 

 a variable objective as manufactured by 

 Zeiss. Its range of magnification lies be- 

 tween 29 and 43 diameters and its free 

 workin & distance between the limits 53 

 millimeters and 13 millimeters. To obtain 

 a similar range with non- variable objectives requires four or 

 five. Variable objectives do satisfactory work and are rela- 

 tively inexpensive. 



A measure of the quality of an objective lies in its ability to 

 make clear any fine and delicate details of structure. It is, 

 therefore, customary to speak of the resolving power of objectives 

 and express this attribute in terms of the number of fine lines per 

 unit length the different objectives will render distinctly visible, 



1 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 trans- 

 mit 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 mono- 

 brom naphthalene immersion objectives since only a part of the full aperture 

 will be available. 



