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, arid 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 e<jnl 

 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 diwyent, arid 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. 1.5, the}' will then 



FIG. 15. Rays 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 ri/'ttnil 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 rlrtn.aL 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 rirtini/ />rinci/>til focus of the lens. 



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

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

 single local 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 IK show the refraction of rays of monochromatic 

 1 A fro/ iniii^c ca.ii l>c received on a screen, but ft virtual image cannot. 



