FORMATION OF A : VIRTUAL IMAGE' 25 



and as there is no focal point, there can be no screen image. 

 Thus two rays radiating from the point A of the object A B fall 011 

 the lens and are refracted in the directions A F, AH: these are 

 divergent and will never meet ; but if the human eye is placed near 

 the lens, so that it can receive the rays F and H, the rays will be 

 converged by the lens of the eye, and will be brought to a focal 

 point in the retina. 



Similarly, from every point in A B there will be a corresponding 

 retinal point. Now if we produce F and H backwards (see the 

 dotted lines in the figure) we shall .find that they intersect at the point 

 C. As the rays F and H are precisely identical with rays which 

 would have diverged from the point C had it been an entity, the 

 retinal image therefore will be an image of a non-existent picture 

 CD. 



The method of drawing this is exactly similar to that of the 



FIG. 27. The formation of a ' virtual image. 



preceding figure. The rays A F and A H are traced through the 

 lens, and their prolongation backwards (see the dotted lines in the 

 figure) gives the point C. Also, as in the preceding figure, any 

 point of the picture can be found by tracing one ray, such as K ; 

 then the intersection of its backward prolongation with a straight 

 line joining B with the optical centre, produced, will give D. 



The points C and D are called the virtual conjugate foci of A 

 and B respectively. In mathematical optics it appears as a negative 

 quantity which satisfies an equation, and is a sort of metaphysico- 

 mathematical truth. In this case the virtual image is convex 

 towards the lens. 



Fig. 27 illustrates the action of a simple microscope. The object 

 itself is not seen, but the picture presented to the eye is an 

 enlarged ghost of it. As some eyes can take in rays of less diverg- 

 ence than others, it might happen that the rays C F, C H, were too 

 divergent for the observer's eyesight, in which case the lens would 



