12S HISTORY OF OPTICS. 



formula not exactly the same as Professor Powell, found also an 

 agreement between these and Fraunhofer's observations. 



It may be observed, that the refractive indices observed and 

 employed in these comparisons, -were not those determined by the 

 color of the ray, which is not capable of exact identification, but those 

 more accurate measures which Fraunhofer was enabled to make, in 

 consequence of having detected in the spectrum the black lines which 

 he called B, C, D, E, F, G, H. The agreement between the theoreti- 

 cal formulae and the observed numbers is remarkable, throughout all 

 the series of comparisons of which we have spoken. Yet we must at 

 present hesitate to pronounce upon the hypothesis of finite intervals, as 

 proved by these calculations ; for though this hypothesis has given 

 results agreeing so closely with experiment, it is not yet clear that 

 other hypotheses may not produce an equal agreement. By the 

 nature of the case, there must be a certain gradation and continuity in 

 the succession of colors in the spectrum, and hence, any supposition 

 which will account, for the general fact of the whole dispersion, may 

 possibly account for the amount of the intermediate dispersions 

 because these must be interpolations between the extremes. The 

 result of this hypothetical calculation, however, shows very satisfacto- 

 rily that there is not, in the fact of dispersion, anything which is at all 

 formidable to the undulatory theory. 



11. Conclusion. There are several other of the more recondite 

 points of the theory which may be considered as, at present, too un- 

 decided to allow us to speak historically of the discussions which they 

 have occasioned.''' 4 For example, it was conceived, for some time, that 

 the vibrations of polarized light are perpendicular to the plane of 

 polarization. But this assumption was not an essential part of the 

 theory ; and all the phenomena would equally allow us to suppose the 

 vibrations to be in the polarization plane ; the main requisite being, 

 that light polarized in planes at right angles to each other, should also 

 have the vibrations at right angles. Accordingly, for some time, this 

 point was left undecided by Young and Fresnel, and, more recently, 

 some mathematicians have come to the opinion that ether vibrates in 

 the plane of polarization. The theory of transverse vibrations is 

 equally stable, whichever supposition may be finally confirmed. 



We may speak, in the same manner, of the suppositions which, from 



24 For an account of these, see Professor Lloyd's Report on Flrjsical Option 

 (B:it. Assoc. Report, 1834.) 



