( 65) 



the errors of' the readiiigs are less than Viooo of tlie deviation, 

 very different intensities may be determined witla the same resistance 

 to at least Vio percent. In this case the current was weak, and it 

 is clear, how with the quadrant-electrometer we may determine 

 amoni,^ others the constant of mirror galvanometers without any 

 difficulty. If on the other hand we have a „Normal Widerstand" 

 of 0.001 ohm, as it is constructed by Siemens and Halske for 

 instance, a current of 1000 ampere may be measured with the same 

 accuracy. 



To conclude I shall point out a circumstance which may occur 

 in some cases with such measurements, and which may cause very 

 great mistakes, and has rendered a great many of my own experi- 

 ments worthless. In this laboratory 30 accumulators are coupled 

 in series, and different groups may be used in different rooms ; in 

 my experiments the current, which passed also through the resis- 

 tance, mentioned on page 6, was taken from a group of 5 accumu- 

 lators; one end of this resistance was connected with the earth, 

 the other with the electrometer ; when however in another room 

 another group of accumulators is used at the same time and part 

 of the circuit is in connection with the earth in that room too, or 

 is not quite insulated, the end, connected with the electrometer may 

 be between two points which are in connection with the earth, and 

 the true difference of potential is not measured. A mistake of this 

 kind is at once found when we change the direction of the current ; 

 moreover this mistake may be easily avoided by insulating the 

 cells used. 



Physical Laboratory. Groninghen. 



Physics. — T,On the influence of the dimensions of the source of 

 ligld in diffraction php.nomena of Fresnel and on the dif- 

 fraction of X-rays." (Third communication.) By Dr. C. H. 

 Wind (Communicated by Prof. H. Haga). 



17. In my former communications on this subject'), I have 

 pointed out (cf. Arts. 10 and IG) that the theory concerning the influence 

 of the widening of the illuminated slit, in the simple form at least 

 in which it was given there, cannot explain the fact that, when 

 the diffraction slit gradually narrows towards one end, the two 

 principal maxima continue to appear as two distinct bright or (on 

 photographic negatives) dark lines, even after the point of inter- 



') Versl. K. A. v. W. 5, p. U8 and 6, p. 7'J, 1897. 



