16 



VEGETABLE HISTOLOGY. 



FORMATION OF AN IMAGE. (1) By double convex lens. 

 Without describing the geometric construction, the following is 

 the fact : That when a small object is placed near the principal 

 focus but a little distance in front of it. the image formed is at 

 a great distance, is inverted and much larger, and that in pro- 

 portion as the object is nearer the principal focus. This is 

 shown in Fig. 14, where the arrow A represents a bright body 

 and the arrow B its inverted image, much larger and at a great 



B 



Fig. 14. 



distance. C is the focus, the body being a little beyond it. The 

 rays of light coming from every point of A are converged by the 

 lens to a corresponding point in the image B, and the latter is 

 real and can be caught upon a screen held at B. The figure 

 represents what takes place in a compound microscope, as will 

 be shown later. The object and image have the same propor- 

 tion as their distances from the lens. 



When the object is very near the focus, but between it and 

 the lens, the rays from the various points of the object are not 

 converged to a corresponding point as in the previous case, but 

 pass through the lens still diverging, but less so than before, 

 and in such a way that if they were prolonged backward they 

 would meet and form an image behind the object on the same 



B_ 



Fig. 15. 



side of the lens as the latter. Fig. 15 will illustrate. An eye 

 held in front of the lens would see the light coming from the 

 object A as if it came from B, and B, therefore, is the image of 

 A, erect, larger, but unreal or imaginary. 



A lens used in this manner to form, with the help of the eye, 

 an erect magnified image, constitutes a simple microscope or 

 magnifier. In both cases considered, the magnification is 

 greater according as the object is nearer the focus and the focal 

 length is decreased, i. e., the lens is more convex. Magnification 



