4BÓ 



mittecl to roininiinirate the general result tiiat for water ilu're exists 

 a dispersion of Frksnei/s coefficMnt and that forinida (3) and there- 

 fore the third term of Lorkxtz is essentia I hi correct. 



I wish to record here my thanks to Mr. W. dk (troot phil. nat. 

 oand. and assistant in the physical laboratory for his assistance 

 during my experiments with the final apparatns. 



The difficnities encountered in these experiments were only snr- 

 mounled after two reconstrnctions of the apparatns. Great annoyance 

 gave the inconstancy of the interference fringes, when the pressnre 

 of the water or the direction of tlow were changed. Then not only 

 the width of the interfereice hands, but the inclination of the fringes 

 were undergoing uncontrollable variations. All these defects were 

 perfectly eliminated by the nse of wide tidies and l)y arranging the 

 end plates in the manner indicated in Fig. 3. 



Fig. 3. 



I am indebted to Mr. J. v.\K DEr ZwaaL, instrumentmaker in the 

 laboratory for his carefully carrying out my instructions and designs 

 in the mechanical construction of the apparatus. 



In fig. 2A a side aspect, and in Fig. 2B a horizontal projection 

 of the arrangement on a scale of abont ^U^'^ is given (see Plate). 



The interferometer is at the right side, at the left the rectangular 

 prism is placed. 



The mounting of this prism is only sketched and was in reality 

 more stable than might be inferred from the drawing. 



