1835.] Scientific Intelligence. 463 



it and printing it, at his own charge :" that is to say, shall, at his 



own charge of whatever trouble or minor expense may be incidental 

 to it, undertake the superintendence of the printing. 



Professor Powell communicated a paper on the present state of 

 the question respecting the theory of the dispersion of light. 



The grand optical discovery of Newton referred to the unequal 

 refrangibility of those primary elements of light, which were desig- 

 nated generally as the red, blue, &c. rays. Each of such species 

 of light was observed to have its refrangibility different, and increas- 

 ing from the red to the blue end of the scale. Newton's successors 

 soon found that the refrangibilities were not only different among 

 themselves for the parts of the spectrum formed by a prism of the 

 same substance, but followed no perceptible proportion, when com- 

 pared from one medium to another. The only attempt to estimate 

 them numerically, consisted in finding the mean refractive index for 

 the substance, and inferring the indices of extreme rays from the 

 amount of separation or dispersion observed. Thus up to a late 

 period, the indices were only determined for the red, blue, and mean 

 rays vaguely and without exact definition ; as it depended only on 

 the judgment of the eye to say how far the red (e. g.) should be con- 

 sidered to extend, and where the yellow should begin ; and what 

 point of the red or yellow, &c. should be taken for the point of 

 measurement. Among the refractions observed thus vaguely for 

 different media, no apparent relation or connexion could be traced : 

 and no theory, whether of emission or of undulation, appeared to 

 afford any explanation of the phenomena. Indeed, all comparison 

 between the rival theories on this point might have been spared, 

 since not even the law of the phenomena, nor even an exact know- 

 ledge'of the facts themselves, had been obtained. All theory \ was, 

 therefore, premature. 



More recently the singular fact of finely-marked dark lines being 

 seen to cross all parts of the spectrum (discovered independently by 

 Dr. Wollaston and Mr. Fraunhofer) afforded the means of more 

 accurate measurements. They formed precise points, assuming dif- 

 ferent relative positions for the different media employed : and by 

 means of them, Mr. Fraunhofer determined with the most elaborate 

 precision, the refractive indices for seven principal lines or definite 

 rays, in each of ten different transparent substances. Thus science 

 obtained the first important requisite, without which no satisfactory 

 investigation of causes can proceed, exact numerical data. But the 

 more exact the data, still only the more palpable was the seeming 

 absence of any law. 



Further, Newton had determined the lengths of those intervals or 

 periods (which he called Jits, but the undulationists waves) which, 

 by whatever name they may be called, have a real existence in the 

 nature of light. These are different for the different rays. Newton 

 determined them only for the red, blue, &c. rays in the same general 

 sense as before ; but Fraunhofer measured them accurately for the 

 seven definite rays above mentioned. Here then is another set of 

 numerical data : and the first obvious inquiry towards investigating 

 a law would be, Can any relation be traced between these two sets 

 of data for different rays and different media ? 



