XIX, 1. Rheinberg: Common Basis ofthe Theories ofMicroscopic Vision. 29 



large number of such slots of very small width. It would be imusual 

 to speak of a diffratioii gratiug of 2 or 3 or 4 slots per iuch of 

 ^/^o" or ^/oq" width. But in this chapter I have endeavoured to 

 show the action of a plane wave front of liglit on any number of 

 slots from two upwards , tracing the eftect of the larger number 

 from that of the smaller, the action of the two ouly having been 

 iirst deduced from that of a Single slot. Moreover no restriction 

 has been implied as to width. It would be quite correct therefore, 

 if we please, to designate all rows of slots from two upwards as 

 diftraction gratings ; whether 2, 3, or 3000, and no matter what 

 Avidth, we have seen that they all foUow the same law, and the 

 action of one evolves itself from the action of the othej. I invite 

 attention to this as it is one of those matters which have created 

 some confusion on the subject of microscopic vision. 



Chapter IV. 

 On Obliquity of Inoidence and Cones of Liglit. 



AaDQ.^P'DP' 



In the previous chapter we have considered the diffractive action 

 of lenses and gratings when the wave front is normal to their plane 

 like the wave front Ä in figure 34. But if the wave front fall upon 

 the slot obliquely as shown by B it ar- 

 rives with a difference of phase at the 

 various points in the plane of the slot, the 

 ditference being measured by the distance 

 WX. The result is that the position of the 

 Chief image and maxima get shifted to DP' 

 and DQj the former always being perpeu- 

 dicular to the wave front, and the first 

 maximum such that IV X -\- J\II^ = ^ 

 wave length. Now if the position of the 

 iutensity curve gets shifted according to the 

 obliquity of the incident wave front, then 

 when we have a wave front of very many 

 degrees of obliquity or in other words a 



converging cone of light, there is an overlapping of all the intensity 

 curves and such au overlapping causes an evenly illuminated central 



