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



at BR etc.^ From C the light reaches the extremities of the slot 

 with a whole wave length difference and tlie iraages are at CP\ 

 CQ^ CE, etc. From this we see that the amplitude in any direction, 

 due to one point is reinforeed by the light due to certain other points, 

 and diminished by that due to certain others. Since however there 

 falls in the direction of e a c h of the rays of the illnminating cone, 

 one Chief Image, one first maximum, one first minimum etc. there is 

 always the same total resultant of light in each of these directions. 

 As soon as we get to directions more oblique than those of the 

 rays proceeding from the illnminating cone we still get maxima and 

 minima formed by some of the rays interacting and producing a cer- 

 tain resultant light, but that resultant is in every case much feebler 

 owing to the absence of its most intense component viz.: the chief 

 image. And as we get to more and more oblique directions fewer 

 and fewer of the rays of the illuminating cone give rise to any 

 etfect there, for we have already seen that the light intensity of the 

 maxima diminishes very rapidly, and is inappreciable after the first 

 few. So that again we see how the cone ot light converging on 

 the slot produces on the other side of it an evenly illuminated surface 

 of angular extension very nearly equal to itself^ surrounded by a 

 surface of unequal luminosity. 



We may now note that so far as the evenly illuminated portion 

 of the image is concerned the effect produced corresponds precisely 

 to that which would have resulted if the slot itself were seif lumi- 

 nous, whenever therefore, we may be dealing solely with this evenly 

 illuminated portion, we shall be perfectly justified in treating of the 

 action of the slot itself as being seif luminous. But if we have to 

 deal with other portions of the cone besides, or with the latter only. 



^) The word ray is here and elsewhere used simply to indicate the 

 direction of the light, and it is to be borne in mind that we are always 

 really dealing with wave fronts proceeding from the points on the grand 

 wave front. But after the investigations in chapters II and III, we need 

 not make the diagrams more complicated, by depicting these other than 

 by Single line or „ray" nor need the action of the slot as a whole be 

 shown other than by „rays" proceeding from its central point. 



-) It is not quite equal to the illuminating cone because each ray 

 gives maxima and minima on both sides of it, and in the direction of its 

 outermost rays, there are of course wanting in the resultant, the maxima 

 or minima which, had there been incident rays of still greater obliquity, 

 would have been formed by these. Moreover the illuminating cone itselt 

 must have slightly less luminous edges for a similar reason. 



