image. The crossing point of these lines determines the position of the cor- 

 responding part of the image. Commencing with any point of the object the 

 corresponding point of the image may be determined as just described, and 

 conversely commencing with the image, corresponding points of the object 

 may be determined. 



FIGS. 14 AND 15. 14. Convex 

 lens showing the position of the 

 object (A-B) outside the principal 

 focus (F), and the course of the 

 rays in the formation of real 

 images. To avoid confusion the 

 rays are drawn from only one 

 point. 



A B. Object outside the prin- 

 cipal focus. B' A'. Real, en- 

 larged image on the opposite side 

 of the lens. 



Axis. Principal optic axis, 

 f, 2,3. Rays after traversing the 

 lens. They are converging, and 

 consequently form a real image. 

 The dotted line and the line (2) 

 give the direction of the rays as if 

 unaffected by the lens. (F). The 

 principal focus. 



FIG. 15. Convex lens, show- 

 ing th'e position of the object (A B) 

 within the principal focus and the 

 course of the rays in the formation 

 of a virtual image. 



A B. The object placed between the lens and its focus ; A' B' virtual 

 image formed by tracing the rays backward. It appears on the same side of 

 the lens as the object, and is erect ($ //). 



Axis. The principal optic axis of the lens. F. The principal focus. 



i, 2, j. Rays from the point B of the object. They are diverging after 

 traversing the lens, but not so divergent as if no lens were present, as is shown 

 by the dotted lines. Ray (/) traverses the center of the lens, and is therefore 

 not deflected. It is a secondary axis ($6). 



SIMPLE MICROSCOPE : EXPERIMENTS 



12. Employ a tripod or other simple microscope, and for 

 object a printed page. Hold the eye about two centimeters from 

 the upper surface of the magnifier, then alternately raise and lower 



