REFRACTION OF LIGHT, 405 



mirror cannot have escaped the notice of the thoughtful 

 pupil. Others will appear. If secondary axes be nearly 

 parallel to the principal axis, well-defined foci may be 

 formed upon them, as well as upon the principal axis. A 

 number of these foci may determine the position of an 

 image formed by a lens. 



(a.) The linear dimensions of object and image are directly as 

 their respective distances from the centre of the lens ; they will be 

 virtual or real, erect or inverted, according as they are on the same 

 side of the lens or on opposite sides. 



628. Construction for Real Images. To determine 

 the position of the image of the object AB (Fig. 309), draw from 

 any point, as A, a line parallel to the principal axis. After refrac- 



FIG. 309. 



tion, the ray represented by this line will pass through F, the prin- 

 cipal focus. Draw the secondary axis for the point A. The inter- 

 section of these two lines at a determines the position of the con- 

 jugate focus of A. In similar manner, the conjugate focus of B is 

 found to be at &. Joining these points, the line ab is the image of 



the line AB. 



. 



629. Diminished Real Image. If the object 

 be more than twice the -focal distance from the convex 

 lens, its image will be real, smaller than the object and 

 inverted (Fig. 310). Construct the image as indicated in 

 the last paragraph. 



