IMAGES BY LENSES. 



If a radiant point be placed at A, fig. 23, at the principal focus 

 of a lens, the rays diverging from it after passing through the 

 lens will be rendered parallel, as may be shown experimentally by 

 receiving them upon a screen as indicated in the figure. An 



Fig. 23. 



illuminated disc will be produced upon the screen equal in size 

 to the lens. 



34. Having explained the change of position which the image 

 undergoes by removing the object indefinitely from the lens, let 

 us now consider how its position will be affected if the object be 

 moved indefinitely towards the lens. 



It is evident, from what has been already explained, that when 

 a very distant object approaches the lens, no change whatever in 

 the position of its image is at first produced, the image remaining 

 always at the principal focus, but the magnitude of the image will 

 be sensibly augmented, its linear dimensions increasing in exactly 

 the same proportion as the distance of the object from the lens 

 decreases. 



When, however, the object has approached within a certain 

 limit of distance, the image will begin, at first very slowly, and 

 afterwards more rapidly, to recede from the lens. It will thus 

 continue to recede, and at the same time to increase in its dimen- 

 sions, until the object is brought to a distance from the lens equal 

 to its focal length. The image having then augmented indefinitely 

 in magnitude and distance, will altogether disappear. 



This is, therefore, an exceptional position of the object, in which 

 no optical image is produced by the lens. 



If we suppose the object to be brought still nearer to the lens 

 than its focal distance, no actual optical image will be produced, 

 but the rays of light which, having issued from the various points 

 of the object, pass through the lens, will be refracted by it into 

 directions such as they would have had if they had issued from a 



H 2 99 



