CONCAVE LENSES. 



479 



length of the latter, generally denoted by a negative 

 sign, so as to distinguish between the action of the two 



FIG. 275. 



kinds of lenses, viz., converging or convex, and diverg- 

 ing or concave lenses. 



A concave lens produces always a virtual erect image, 

 smaller than the object, whatever the distance of the 

 object. Fig, 276 serves to explain the formation of such 

 an image. The ray A c from A proceeds, after having 

 passed through the lens, in the direction cd, as if it 

 originated in / ; the ray A e, through the centre of the 

 lens, passes to (/without changing its direction ; c d and 



FIG. 276. 



e g appear both to come from a : hence a is the image of 

 A. Similarly b is the image of J3, as seen by tracing 



