VIRTUAL AND REAL IMAGES. 



that the nearer the object be placed to the principal focus 

 of the lens (F) for parallel rays, the larger will the virtual 

 image appear. To yield a virtual image, the object must 

 be placed between the lens and its principal focus. 



FIG. 3. 



A real image is formed from a double convex lens, only 

 when the object is outside, or in front of, the principal focus 

 for parallel rays ; this image is inverted, and may be studied 

 by receiving it upon a screen of ground glass. The action 

 of lenses in the production of real images should be care- 

 fully considered by the student, as upon it depends chiefly 



FIG. 4. 



the results of his manipulation, which may be perfect or 

 defective in proportion as he understands the theory of 

 this simple optical instrument. 



An inspection of Fig. 4 will show the two consequences of 

 this action. If A B represents a lighted candle placed far 

 distant from the principal focus of the lens, a small, real 



