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1905-1906.] A ddress by the P reside n t. 331 



trary, cannot be received on a screen, — they can only be 

 received on the retina of the eye. The diagram on Plate 

 XXVII., Fig. 1, vpill enable this to be understood. 



Let P be the focal point of a lens F K, and A B an object 

 placed at a distance farther from the lens than the focal point : 

 rays of light will emanate from every part of A B towards the 

 lens, and, emerging from it after refraction, will pass on. 

 Let us follow four of these rays. The rays A F and A H 

 will, by the laws of refraction, meet at C, where it will form 

 an image of the point A. In like manner the ray B K will 

 proceed to D, and as we have seen that the rays which pass 

 through the optical centre of a lens suffer no refraction — the 

 other ray proceeding from the point B and passing through 

 the centre of the lens will go straight to the point D, where it 

 will unite with the ray B K and form at D an image of the 

 point B. In like manner the rays proceeding from the other 

 parts of A B will meet at various points between D and C, 

 and thus a complete enlarged image of A B will be formed at 

 C D. It is to be observed that the image is inverted, and is 

 also concave towards the lens : if the object had been concave 

 towards the lens, then the image would have been straight. 

 It is, however, a real image, and could be received on a screen 

 or viewed hy another lens. 



Let us see now, from Fig. 2 on Plate XXVII., how a 

 virtual image is formed. In this diagram let A B be an object 

 placed nearer to the lens than its focal point P. Pays of 

 light will likewise emanate from it towards the lens, but after 

 emerging, instead of being convergent they will be divergent, 

 and consequently the rays proceeding from the same point of 

 the object will never meet. If, however, an eye be applied to 

 them at such a distance from the lens that it can receive the 

 whole of the emergent rays, an image of the object will be 

 formed upon the retina, and an enlarged virtual image will 

 thus be seen at C D in the direction of the dotted lines. It 

 will be observed that the virtual image, although curved, but 

 in the opposite sense to the real image, is yet not inverted. 



Having now got some notion , of the various properties of 

 lenses, and of the formation by them of images real and 

 virtual, we are in a position to consider their combination in 



