( i:^ ) 



of the line. The same experiment may be made in the case of 

 fig. 9 by covering somewhere the transitionzone (for the greater 

 part) with a sharply ontlir.ed piece of paper. That the disappearing 

 of the line is really due to the disappearing of the transition-zone and 

 not to ordinary contrast in consequence of the covering piece 

 of paper will appear if we take first a dark piece of paper and 

 afterwards a light one, both producing the same eff'ect. 



9. Although the optical illusion described above may be produced 

 especially by the methods given here, yet it may be often observed 

 in a simpler way. In nearly all cases where we have an umbra 

 passing into a penumbra we rnay observe a dark and a bright line 

 as borders of the penumbra, and the optical illusion appears in all 

 the various forms described above and even more. 



1(). Attention has been drawn to the appearing of bright and 

 dark lines conjointly with a penumbra, as described in 7 and 8 by 

 Sagnac (Journal de Physique VI p. 169, 1897). Formerly I did 

 not conclude from, what he said on this subject that he took those 

 lines for the result of optical illusion only, although now it seems 

 to me, if not probable, yet possible that this has been the case. 

 That I feel justified to communicate my observations on this subject 

 somewhat extensively is owing to the great importance that, in my 

 opinion, must be attached to this optical illusion. 



In the first place it has deluded many a physicist, who thought 

 that he observed diffraction lines or other important lines in cases 

 where now it is certain that no real maxima or minima of bright- 

 ness of any importance existed; so it lias especially caused bright 

 and dark lines to be observed in A'- rays- shadows the true nature 

 of which some have tried to explain in various ways, but no one 

 had yet sufficiently accounted for. (Another time I hope to return 

 to this subject). 



In the second place it seems to me that this optical illusion may 

 under special conditions lead to the observation of the doubling of 

 bright or dark lines or l)ands where in leality there is nothing but 

 a broadening of these lines or bands together with the borders 

 growing less sharply defined (compare what has been said in 5. sub 

 5°), a thing that may occur t. i. when the optical system we use in 

 observing is not accurately adjusted (resp. accomodated). It may 

 be that in some cases this optical illusion must be held responsible 

 for doublings which have been observed and which have not yet 

 been explained in the right manner or not yet been explained at all. 



Ill the third place the optical illusion may lead to errors in the 

 estimation of the place of the maxima and minima of brightness 



