22 



course of rays from the object it will be noted how 

 they are converged and refracted by the lens and 

 intercepted by the pup.il of the eye. If now the 

 lines between c and e be considered elongated, 

 they will be found to meet beyond a b and there 

 form a virtual image. It will be well to under- 

 stand at this point the difference between a real 

 and a virtual image. The-raz/ image is one which 

 can be accurately seen and projected upon a sur- 

 face, as in the magic lantern, or in the picture 

 which is given by the photographic camera. The 

 virtual image cannot be so shown. In a lens 

 of less convexity or longer focus, there is less 

 convergence of rays and the length of the virtual 

 image is consequently reduced and thus the mag- 

 nification is less. 



Spherical Aberration. In considering the 

 refraction of light by a lens up to this point, we 

 have purposely avoided mentioning another quality 

 which is incident to it. In magnifying an object 

 by a single lens it will be noticed that the virtual 

 image as seen through its central portion is quite 

 clear, while that near the margin or edge is quite 

 indistinct. This is due to spherical aberration and 

 the extent of this aberration increases with the 

 power of the lens. It is due to the unequal refrac- 

 tion between those rays passing near the margin 

 and those passing through the central portion, so 

 that the rays instead of combining at the focal 



