Appendix A: The Microscope and Its Optical Principles 137 



The Compound Microscope. The general principle of the com- 

 pound microscope is represented in Fig. 48. The object ab 

 lies beyond the principal focus of the first lens or objective (really 

 a system of lenses), hence the image AB is reversed. This 

 image, in turn, is viewed through a lens, the eyepiece or ocular 

 situated nearer the eye of the observer. The ocular acts as a 

 simple magnifier, projecting and enlarging the image but not 



A 



a 



B 



FIG. 48. 



reversing it again. As a matter of fact, the ordinary ocular of a 

 compound microscope cannot be taken from the instrument and 

 used as a simple magnifier because it is made of two planoconvex 

 lenses which are so adjusted that the image from the objective of 

 the compound microscope is not brought to focus until it has 

 traversed the larger or field lens of the eyepiece (Fig. 52). The 

 image is really examined, therefore., at a point between the two 

 lenses of the eyepiece. Such an eyepiece is termed a negative 

 eyepiece or ocular and is widely used today for microscopical work. 

 Positive eyepieces are made, however, and they may be used as 

 simple magnifiers when removed from the compound microscope. 

 A good objective is made up of from two to five systems of 

 lenses as shown in Fig. 49. A single system in turn may be a 



