22 



BACTERIOLOGY 



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 

 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 



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



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 

 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 



Pig. 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 Pig. 8. 



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 micro- 

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

 8. Fig. 9 shows the change which is introduced by the use of 

 an eye-piece of higher magnifying power. 



It will be noted that the objective and lower lens of the eye- 



