CHAPTER II 



USE OF THE COMPOUND MICROSCOPE 



Classification of Useful Magnifications. — The meaning 

 of aperture was given in the last chapter. The aperture 

 method may be applied to measuring the useful cone of 

 Ught falUng on an object, as well as to measuring the cone 

 of light admitted by the objective from the object. (Com- 

 pare Figs. 6 and 7.) The average of these two apertures 



0.3 



0.65 



1.25 

 Fig. 7. 

 Fig. 6. This shows the objective aperture circles and the condenser aperture 

 circles seen on the back of objectives of the five named apertures, with a con- 

 denser cone of a maximum aperture of 1.25. The radii are made proportional 

 lo the apertures. 



Fig. 7. — Diagram showing upper lens of water-immersion condenser and front 

 lens of each of five objectives of apertures as in Fig. 6. The maximum condenser 

 cone has an aperture of 1.25, of which the limiting rays are shown. 



is the working aperture. The numerical value of the 

 working aperture is the most important figure in practical 

 microscopy. One thousand times the working aperture 

 forms the maximum useful magnification. Hence a classi- 

 fication of magnifications is a classification of working 

 apertures. For the purposes of this book, the following 

 classification is offered. (For immersion objectives and 



26 



