KELATIVE TO PHYSICAL OPTICS. 



6*5 



some property vvliit^h the drops retain, whilst 

 they are in the upper part of the air, but lose 

 as they come lower, and are more mixed with 

 •one another." Phil. Trans. XXXII. 243. 



From a consideration of" the nature of the 

 secondary rainbow, of 54°, it may be in- 

 ferred, that if any such supernumerary co- 

 lours were seen attending this rainbow, they 

 would necessarily be external to it, instead 

 of internal: and Mr. Dicquemare has actually 

 recorded an observation of this kind. The 

 circles, sometimes seen encompassing the 

 observer's shadow in a mist, are perhaps 

 more neai'ly related to the common colours 

 of thin plates as seen by reflection. 



IV. AKGUMENTATIVE INFERENCE RESPECT- 

 ING THE NATURE OF LIGHT. 



The experiment of Grimaldi, on the 

 crested fringes within the shadow, together 

 with several others of his observations, equal- 

 ly important, has been left unnoticed by New- 

 ton. Those who are attached to the New- 

 tonian theory of light, or to the hypotheses of 

 modern opticians, founded on views still less 

 enlarged, would do well to endeavour to 

 imagine any thing like an explanation of 

 these experiments, derived from their own 

 doctrines; and, if they fail in the attempt, 

 to refrain at least from idle declamation 

 against a system, which is founded on the 

 accuracy of its application to all these facts, 

 and to a thousand others of a similar nature. 



From the experiments and calculations 

 which have been premised, we may be al- 

 lowed to infer, that homogeneous light, at 

 certain equal distances in the direction of its 

 motion, is possessed of opposite qualities, ca- 

 pable of neutralising or destroying each other, 

 and of extinguishing the light, where they 



happen to be united ; that these qualities 

 succeed each other alternately in successive 

 concentric superficies, at distances which 

 are constant for the same light, passing 

 through the same medium. From the agree- 

 ment of the measures, and from the simi- 

 larity of the phenomena, we may conclude, 

 that these intervals are the same as are con- 

 cerned in the production of the colours of 

 thin plates ; but these are shown, by the ex- 

 periments of Newton, to be the smaller, the 

 denser the medium ; and, since it may be 

 presumed, from the impossibility of imagining 

 any way in which their number can be 

 changed, that it must necessarily remain un- 

 altered in a given quantity of light, it follows 

 of course, that light moves more slowly in 

 a denser, than in a rarer medium : and thiri 

 being granted, it must he allowed, that re- 

 fraction is not the efltct of an attractive force 

 directed to a denser medium. The advocates 

 for the projectile hypothesis of light must 

 consider, which link in this chain of reason- 

 ing they may judge to be the most feeble; 

 for, hitherto, I have advanced in this paper 

 no general hypothesis whatever. But, since 

 we know that sound diverges in concentric 

 superficies, and that musical sounds consist 

 ' of opposite qualities, capable of neutralising 

 each other, and succeeding at certain equal 

 intervals, which aredifterent according to the 

 difference of the note, we are fully authorised 

 to conclude, that there must be some strong 

 resemblance between the nature of sound and 

 that of light. 



I have not, in the course of these investiga- 

 tions, found any reason to suppose the pre- 

 sence of such an inflecting medium, in the 

 -neighbourhood of dense substances, as I. was 

 formerly inclined to attribute to them; and, 



