22 BACTERIOLOGY 



fraction is most evident when the surfaces of the lens are rough- 

 ened by scratches or by presence of dust, but even the most per- 

 fect lens systems are not wholly free from diffraction phenom- 

 ena. Some of these defects will require brief consideration in 

 our discussion of the compound microscope. 



In the modern compound microscope the beam of light pro- 

 ceeding from a point in the object is refracted by the lens system 



FIG. 8. Image formation in the compound microscope. Compare with Fig. 9 



of the objective (Fig. 6) so as to render the rays slightly conver- 

 gent. Near the upper end of the tube of the microscope these 

 rays are further refracted by the lower lens of the eye-piece and are 

 converged and brought to a focus in the interior of the eye-piece. 

 A screen placed at this level would show a real image, and any 

 pattern (for example an eye-piece micrometer) inserted in the 



FIG. 9. Image formation in the compound microscope with an eye-piece of 

 higher power. Observe that the increased magnification is accomplished by narrow- 

 ing the beam of light which enters the eye and so diminishing the size of the closing 

 angle. Compare with Fig. .8. 



eye-piece at this level is readily fused with the microscopic field. 

 Continuing in a straight line the rays diverge from this focus to 

 reach the upper lens of the eye-piece. In traversing this lens 

 they are again refracted and made parallel so that they will 

 enter the eye and be brought to a focus on the retina. The paths 

 of two beams of light, one proceeding from the center of the mi- 

 croscopic field and one from its periphery, are illustrated in Fig. 



