1835.] Scientific Intelligence. 147 



our silver ; and that of Corinth, or Syracuse, from the analysis of a 

 didrachma of that coinage, to have been about equal to the lowest 

 value of the Attic. The Attic currency was shewn to have been 

 at one time of a little lower standard than it was afterwards raised 

 to, and then again to have been much depreciated, till it was less 

 fine than the average standard of Greek silver, and the early Roman. 

 And it was inferred that, although the ancient money was generally 

 of a very high standard of fineness, the alloy found in it was pur- 

 posely mixed, and not owing only, to want of skill in refining. 



Mr. Powell, on presenting a copy of his paper from the Journal 

 of Science, entitled " An Abstract of M. Cauchy's Theory of Undu- 

 lations, leading to an explanation of the dispersion of light, &c., made 

 a few remarks on the nature of the results he had recently obtained, 

 and which will appear in the next part of the Philos. Transactions. 

 In a former communication (see Records of General Science, I. 463) 

 he had explained generally the state of the question at issue ; and 

 now proceeded more particularly to refer to the nature of the spectra 

 formed in the very remarkable experiment of M. Fraunhofer, by 

 covering the object glass of a telescope with a fine grating of parallel 

 threads, and viewing through it a narrow line of light parallel to the 

 threads. The appearance is that of the image of the line colourless 

 in the centre, and a spectrum or coloured image of it on each side. 

 In these spectra the colours are perfectly pure. They are produced 

 simply by the interferences of the rays passing the grating, which 

 give bright and dark spaces, at different intervals, for the different 

 rays ; and at such intervals as allows the points of brightness for the 

 red, yellow, green, &c. to succeed one another in such a manner as 

 to give a very complete insulation of each coloured ray in a state of 

 great purity. Professor Airy considers this as the natural, or 

 standard, spectrum ; from which those formed by prismatic refraction 

 deviate in difi^erent ways, according to their peculiarities of dispersive 

 character. The intervals between successive definite rays in the 

 interference spectrum are precisely proportional to the differences of 

 the lengths of their waves, or periods. 



It is by means of the data derived from these spectra by M. 

 Fraunhofer, as compared with those of the refraction-spectra, that 

 the results of the author's calculations have been obtained. By his 

 extremely perfect apparatus M. Fraunhofer was able to obtain these 

 spectra in such a state of purity that the dark lines were visible. 

 Thus, by accurate micrometric measurement, he determined the 

 lengths of the waves for each of the rays defined by the dark lines. 

 These values are given in Sir J. Herschel's treatise on Light, art. 751. 



M. Cauchy's theory gives rise to the establishment of a relation 

 between the length of a wave and the velocity of its propagation. 

 On the ordinary theory no such relation subsists : and by conse- 

 quence, rays whose waves are of all lengths ought to be refracted 

 alike. Thus M. Cauchy's theory assigns an explanation of the un- 

 equal refrangibility. From his memoirs, the author has deduced a 

 formula expressing the precise nature of this relation. On any 

 theory of undulations the velocity is the reciprocal of the refractive 

 index. In this point of view the formula deduced from the theory 

 (which ought to give the value of the refractive index (^) for a ray 



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