496 HISTORY OF OPTICS. 



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 cer- 

 tain gradation and continuity in the succession 

 of colours in the spectrum, and hence, any sup- 

 position 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, how- 

 ever, shows very satisfactorily 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 undecided to allow 

 us to speak historically of the discussions which 

 they have occasioned 24 . For example, it was con- 

 ceived, for some time, that the vibrations of po- 

 larized light are perpendicular to the plane of 

 polarization. But this assumption was not an es- 

 sential part of the theory ; and all the phenomena 

 would equally allow us to suppose the vibrations 



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

 Physical Optics. (Brit. Assoc. Report, 1834.) 



