226 M. W. Weber on the Measurement of Electric 



or harmonic colour*." To this might perhaps be added, that 

 there is a tendency in the eye to, as it were, decompose white 

 light into two complementary colours ; and further, that the 

 predominant decomposition is into red and green. 



Applying this theory to the phenomena of coloured (e. g. red 

 and green) shadows, the red shadow has already been shown to 

 be simply due to the illumination of a colourless shadow by a red 

 light ; whilst on the whole of the rest of the field of the white 

 screen, the red tint cast from the magic lantern is sufficiently 

 powerful to overcome the green tint which the eye would other- 

 wise perceive, excepting at one spot, namely, that which does 

 not receive any red light on account of the interposition of the 

 opake rod. Here the green (harmonic) colour, having no anta- 

 gonistic red to overcome it, is rendered sensible to the eye. 



XXIX. On the Measurement oj Electric Resistance according to 

 an absolute Standard. By Wilhelm Weber f. 



§ 1. Explanation of the absolute unit of measure for Electric 

 Resistances. 



IF there are measures for time and space, a special fundamental 

 measure for velocity is not necessary ; and in like manner 

 no special fundamental measure for electric resistance is needed if 

 there are measures for electromotive force and for intensity of 

 the current ; for then that resistance can be taken as unit of 

 measure, which a closed conductor possesses in which the unit of 

 measure of electromotive force produces the unit of measure of in- 

 tensity. Upon this depends the reduction of the measurements 

 of electric resistance to an absolute standard. 



It might be thought that this reduction would be more simply 

 effected by reverting to the special dimensions, length and sec- 

 tion, and adhering to that metal {copper) which is best fitted and 

 is most frequently used for such conductors. In that case the 

 absolute unit of measure of resistance would be that resistance 

 which a copper conductor possesses whose length is equal to the 

 measure of length, and whose section is equal to the measure of 

 surface, in which, therefore, besides measure of length and sur- 

 face, the specific resistance of copper must be given as unit for the 

 specific resistance of conducting substances. Thus a special 



* Brewster's ' Optics,' p. 309. 



t Translated from Poggeudorff ' s Annalen, vol. lxxxii. p. 337, by Dr. E. 

 Atkinson. [From the great scientific and practical importance which the 

 determination of electric resistances has of late acquired, it has been thought 

 advisable to give a translation of Weber's original paper published in 185J, 

 containing the method of referring these resistances to an absolute stand- 

 ard. — Eds.] 



