14 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS 



This law forms a ready means of determining the focal length of 

 a lens. An object is placed in front of a lens, and the distances 

 between this object and the lens and a screen to receive the image 

 of the object are so adjusted that the image of the object becomes equal 

 in size to the object itself. The distance of the object from the screen 

 divided by 4 gives the focal length of the lens. 



If a radiant be placed between a lens and its principal focus, the 

 rays on the other side of the lens are still divergent, and will never 

 meet in a focus on that side. This is seen in fig. 1 5 ; but if they are 

 traced backwards, as in the dotted lines of fig. 15, they will then 



FIG. 15. Kays diverge when a radiant is placed between a lens and its 

 principal focus. Focus of divergent rays is virtual. 



meet in a point. This is called the virtual conjugate focus of the 

 radiant. The principal focus of a concave (or diverging) lens is 

 shown in fig. 16. It will be seen that the principal focus is not 

 real but virtual. 1 Parallel rays falling on a concave lens are rendered 



FIG. 16. ' Virtual ' focus of concave lens. 



divergent on the other side of the lens, and consequently can never 

 come to a focus. But if we trace these divergent rays backwards, 

 as in the dotted lines of fig. 16, we find that they meet in a point, 

 and this point is called the virtual principal focus of the lens. 



It will be manifest that since the rays in passing through lenses of 

 various kinds are unequally refracted they cannot all meet exactly in a 

 single focal point. This gives rise to what is a most important feature 

 in the behaviour of lenses, which is known as spherical aberration. 



Figs. 17 and 18 show the refraction of rays of monochromatic 

 1 A real image can be received on a screen, but a virtual image cannot. 



