18 Rheinberg: Common Basis oftheTheories ofMicroscopic Vision. XIX, 1. 



inward nearest P'. An apochromatic lens may be tried after using 

 an ordinary acbromatic objective, to test that there is no perceptible 

 difterence. 



Tbese experiments tben exemplify ocularly tbe formatiou of tbe 

 disc and rings, tbe way tbey vary accordiug to tbe affective aperture 

 of tbe lens, tbe colour effects produced in connection with tbem, and 

 bow overlapping- takes place. 



The reasons \ve have cbosen to work witb slits instead of witb 

 circnlar boles are firstly tbat siifficiently strong ligbt to view tbe 

 effects witb tiny boles is seldom available , and secondly tbat tbey 

 cannot be so easily or exactly regulated as to diameter. It is 

 obvious tbat tbe cross section of tbe effect produced by tbe slits will 

 be nearly tbe same as tbat produced by circular boles and we are 

 tberefore at liberty to use tbe more convenient form. Tbe pro- 

 duction of tbe ordinary cbromatic aberration and spberical aberration 

 effects are too well known to need any demonstration. 



Chapter III. 

 Diffraction and Diffraction Grätings. 



Most tbeories of microscopic vision being based upon diffraction 

 in some form or otber , it becomes desirable to examine some of 

 tbe effects of diffraction and diffraction gratings. 



Diffraction bas been defined by Preston^ to denote tbe pbeno- 

 mena due to tbe mutual interference of disturbances propagated from 

 tbe various Clements of a Single wave. 



In tbe previous cbapter we bave noted bow by narrowing down 

 tbe aperture of a lens , or of a slot witbout any lens , we obtain a 

 widening or broadening out of tbe Image of a luminous line , witb 

 visible spectra violet side inwards, red outside, on eacb side of it ; 

 and we saw bow tbis was due to tbe interference of waves from 

 every poiut of tbe wave front. Tbe effect described is usually said 

 to be due to tbe diffractive action of tbe lens or slot. 



Now instead of cutting off tbe wave front X Y on botb sides 

 so as to leave only a Single slot, we migbt leave two or more slots 

 free as indicated in figure 18. In tbat case we bave manifestly to 



1) Preston, Thomas. The theory of light. London 1895. p. 210. 



