16 PRACTICAL HISTOLOGY. 



focal distance, but outside the principal focal point (F), a 

 real magnified image of the object (A B) is produced. The 

 divergent luminous pencil proceeding from a is brought to 

 a real focus at A. The position of A is determined by the 

 point at which the refracted ray a g intersects the secondary 



FIG. 17. Production of a real magnified image. (Lommel.) 



axial ray a O A : that is, the ray passing from a through 

 the optical centre of the lens. Luminous pencils, of 

 course, proceed from every point of the object a b, and are 

 focalised at corresponding points of A B ; these, however, 

 are for the sake of clearness omitted from the figure. 



Conversely, it will be readily seen that if A B were the 

 object placed at more than twice the focal distance, a real 

 diminished image would be produced at a b. The real 

 images produced by a convex lens are always inverted. 



27. Virtual Image, with the Simple Microscope. 

 If an object A B (Fig. 18) be placed within the focal dis- 

 tance of a convex lens, 

 no real image can be 

 produced, because the 

 rays proceeding from 

 A issue divergingly 

 from the lens as if they 

 came from the virtual 



point a. Although in FlG . zS.-Virtualmagnified image. 



this case the lens cannot 

 focalise the rays proceeding from A B, the crystalline lens 

 of the eye of an observer can do this, and thereby produce a 

 real image of A B on the retina, provided that A B is 

 placed only a little way within the focal distance of the 



