( 71 ) 



of a fiiii-likc broadeniug of the iiuaoe of tlie slit, chiefly manifesting 

 itself in a diverging of the two bright lines of the outer side. If 

 we might assume, that this fan-like broadening really exists and is 

 a consequence of diffraction, and further that such a broadening of 

 the, image does not occur before the diftVaction slit is so narrow as 

 to correspond at the utmost to a value 2 for the quantity ?;, we 

 should find by calculation, as the diffraction slit has proved by 

 measurement to have a width of 2,3 ^/ on the spot where the 

 phenomenon is seen : 



Ax> 0,12 ^« . 



Though on one hand I am aware that what 1 have described as 

 seeming to be a fan-like broadening of the image of the slit is in 

 its appearance so utterly mean, that every one might feel justified 

 in not accepting my interpretation, yet it is on the other hand of 

 some importance, that this particular presented itself to my eye quite 

 spontaneously, I having had not the slightest expectation to observe 

 anything of the kind. 



40. After having observed the phenomenon mentioned under 3" 

 on plate A, I tried to find something of the same kind on plate 

 B, and indeed I succeeded in finding a place which gave an indi- 

 cation of a phenomenon somewhat like it. The phenomenon being 

 even less distinct here than on plate A is in itself not astonishing 

 at all, as plate B on the whole seems to be much less affected 

 by the processus of insolation and development than plate A. 

 The spot where I believed to observe the fan-like broadening lies 

 higher here than on plate A, viz : on such a height as to correspond 

 with a width of the diffraction slit of about 30 /<. 



If the reality of this phenomenon might be accepted, we might 

 deduce from it by calculation: 



Ax ^ 0,15 1.1 u, 



suppositions of the same kind being made as before. 



It is in agreement with the above remarks that the image of 

 the slit extends considerably farther, at the point-end, on plate A 

 than on plate B and less far on both the plates than would corre- 

 spond to the real length of the diffraction slit. We may, however, 

 not attach too much importance to this fact, 1°. because plate B 

 shows on the whole a less intensive darkness than plate A, as has 

 already been said, and 2". because the diffraction slit becomes so 



