824 



PRELIMINARY DIOPTRIC CONSIDERATIONS. 



2. Rays proceeding from a point (IV, 1) in the prolonged principal axis beyond 

 the principal focus (f) , are united at a point on the opposite side of the lens (v) 

 (conjugate focus). The following conditions are possible: (a) If the distance of 

 the point of light from the lens is double the focal distance, the conjugate focus 

 is at an equal distance on the opposite side (double the focal distance). (6) As 

 the point of light moves nearer the lens, the conjugate focus moves further away. 

 (c) If the point of light is more than double the focal distance from the lens, the 

 conjugate focus is correspondingly closer to the lens. 



3. Rays that proceed from a point in the principal axis (III, b) within the 

 focal distance are rendered less divergent, but do not meet again; conversely, 

 rays that converge on a convex lens are united at a point within the principal 

 focal distance. 



4. If the point of light (V, a) lies in a secondary axis (a b) the same laws 

 hold good, provided that the angle made by the secondary axis with the prin- 

 cipal axis is small. 



Formation of Images by Convex Lenses. After what has been said about 

 e conjugate foci of rays proceeding from a point of light, it is easy to construct 

 ic image of an object produced by a convex lens. This is done simply by pro- 

 ting the images of various points of the object. For example (in V) b is ob- 

 viously the image of the point a of the object, v the image of 1; the picture is 

 or.1v o/ e T^K- + Convex . lense s form inverted and real images (upon a screen) 

 1 ob Jects as are situated beyond the principal focus of the lens, 

 regard to the size and distance of the image from the lens, the following 



ft ^ t0 be I i t ^ d: (a) If the ob J ect is d uble th e focal distance from 

 its image is of the same size as the object and at an equal distance from 

 and at t'hp * As . the object approaches the principal focus, the image recedes 

 ^ H ^f V? 16 be comes larger, (c) On the other hand, if the object is 

 JS double the focal distance from the lens, the image approaches the lens, 

 the same time becomes smaller. 



