( «8 ) 



The followiug question seemed rather important. What are the 

 different stadia through which the primary diffraction image^ (pro- 

 jected by the edge of a screen f. i.), the luminous slit being gradually 

 widened, passes into the shadow image, belonging to a slit so wide 

 that diffraction has finished to be of any consequence; and how does 

 the diffraction image present itself to our eye in the different stadia 

 under the influence of the optical illusion mentioned. In order to 

 be able to answer this question I have photographed a series of 

 fifteen diffraction images of a rather wide slit, the luminous slit 

 having throughout the whole series an ever increasing width, but 

 the conditions of the experiment being left quite unaltered in any 

 other respect. The width of the latter slit ranged between such 

 limits, that the space through which the primary diffraction curve 

 must be displaced, in the construction of Art. 5, in order to obtain 

 a correct geometrical representation of the secondary diffraction image 

 we should expect, corresponded in the consecutive experiments to 

 displacements of the starting-point of the effective arch on the 

 spiral of Corku, as by which the well-known quantity t' of Fresnel 

 varied from 0,2 (when the slit was narrowest) to 8,4 (when the 

 slit was widest). Applying the construction alluded to, I have on 

 the other hand drawn common-type graphical representations of 

 the distribution of light in the diffraction images belonging to the 

 consecutive widths of the slit. It is not the place here for a de- 

 scription in details of the distributions of light expected and of 

 the distribution of brightness observed on the negatives ; it may 

 be sufficient to remark that most of the plates of the series 

 showed a considerable difference between the distribution of light and 

 that of brightness, a difference even as concerns the general aspect 

 of the image. There is, however, not a total want of regularity 

 in these differences. I generally found that the eye perceives maxima 

 resp. minima of brightness in those places of the zones of continually 

 increasing intensity (transition zones), where — in consequence of 

 undulations of the primary diffraction curve — the rate of increasing 

 of the intensity changes considerably. For the rest the diffraction 

 appeared to come forth pretty clearly in the first plates of the 

 series (the luminous slit being rather narrow), so far as concerns 

 the general aspect of the images, whereas its influence further on 

 in the series decreases more and more and is at last scarcely 

 recognised. The latter remark is of some importance especially 

 with a view to the following considerations. 



22. The leading idea of the whole investigation has always been 

 the desire to get an estimation of the wave-length of X-rays, or 



