THE RESOLVING POWER OF THE MICROSCOPE. 



23 



them. That is one of the most important generalisations of 

 Abbe, made over thirty years ago. 



As it may not be quite clear to all as to what is meant by 

 dioptric and diffracted beams of light, I would like, without 

 entering into details, to point out that it simply relates to 

 distribution of the light which has got through the object. If 

 the object is a single narrow slot, and a parallel beam of light 

 passes through it, it appears on the other side as a broad beam of 

 varying intensity. The light intensity of the central part of this 

 broad beam might be represented by the curve xy (Fig. 2). 



<%2 



I Slot 



Suppose we have two slots, each as wide as the first one, and 

 a space between them of the same width, then the distribution 

 of the light which has passed through is different. The hump 

 of the intensity curve we had before gets broken up into three 

 humps, a, b, and c (Fig. 3), by the so-called, " interference " 

 of light waves. The central hump b is known as the direct or 

 dioptric beam, and the humps a and c, on either side of it, are 

 called the first maxima. A better name for the central hump 

 than either of the two above is that which Lord Kayleigh has 

 applied to it, viz. the zero maximum. 



What happens if we have three slots instead of two, and pass 



