Ch. I] GEOMETRICAL CONSTRUCTION OF IMAGES 



different refracting medium, or a reflecting 

 surface. That is, the direction of a ray of 

 light may be changed by using a mirror, or 

 by putting in its path a transparent medium 

 of greater or less refracting power. 



(2) The second law is, that objects are 

 always seen in the direction in which the 

 light' reaches the eye, regardless of the act- 

 ual position of the object. This will be 

 abundantly illustrated in the chapter on 

 drawing; and every one knows that objects 

 seen in a mirror are not where they appear 

 to be in the mirror. 



§ 11. Construction of real images. 



— (1) The object must be situated 

 at a greater or less distance beyond 

 the principal focal point (fig. 11). 



(2) From some point in the object, 

 draw a line to the refracting plane of 

 the lens (§ 10) parallel to the principal 

 axis, and from this crossing point at 

 the refracting plane of the lens to the 

 focus of the lens, and continue the line 

 indefinitely (fig. 11). 



(3) From the same point of the ob- 

 ject as in (2), draw a secondary axis 

 through the optical center of the lens 

 and extend it indefinitely (fig- n). 



The image of the point in the object 

 from which the two lines were drawn 

 will be located at the point where the 

 two extended lines cross above the lens 

 (fig. 11). 



The image of all the other points of 

 the object may be determined by 

 drawing lines from them exactly as 

 just described. 



If the image is known one can find 

 the object by reversing the process 

 just described. 



Fig. n-12. 



STRUCTION OF REAL AND OF VIR- 

 TUAL Images. 



Object, Object The object of 

 which an image is to be formed. 



Axis, Axis The principal optic 

 axis extended above and below 

 the lens to the object and image. 



S Axis, S A xis Secondary axis 

 passing from the object through 

 the center of the lens. 



f> /• /- / T ' ie principal foci of 

 the two lenses. 



r-p The plane of refraction 

 (the ideal plane at which all the 

 refraction is made to occur in 

 diagrams of thick lenses). 



A'. Image Real image. 



V. Image Virtual image indi- 

 cated by broken lines as it has no 

 real existence. 



b, r m Rays of light indi- 

 cated by lines passing from the 

 extremities of the object to the 

 extremities of the real image, 

 which is inverted. 



/>, / 2, 3 ./, v m Lines rep- 

 resenting rays of light from the 

 objei 1 passing in a diverging 



manner above the Ions, and ex- 

 tended by broken lines below the 

 lens to form a virtual image at 

 their crossing points, V hi. 



