138* PHYSICAL OPTICS. 



th^t thefe intervals are the fame as are concerned in the 

 produ6lion of the colours of thin plates j but thefe are fliow n, 

 by the experiments of Newton, to be the fmaller, the denfcr 

 the medium ; and, fince it may be prefumed that their num- 

 ber mull neceflarily remain unaltered in a given quantity of 

 Ughtmovfs light, it follows of courfe, that light moves more flowly in a 

 deafer mediums. ^^^^^^' than in a rarer medium : and this being granted, it 

 muft be allowed, that refradion is not the effe6l of an attrac- 

 tive force direded to a denfer medium. The advocates for 

 the projedile hypothefis of light, muft confider which link ir^ 

 this chain of reafoning they may judge to be the moft 

 feeble J for, hitherto, I have advanced in this Paper no ge- 

 tight and found neral hypothefis whatever. But, fince we knpw that found 

 ftrongly refemble diverges in concentric fuperficies, and that mufical founds 

 confid of oppofite qualities, capable of neutralifing each 

 other, and fucceeding at certain equal intervals, which are 

 different according to the diiTerence of the note, we are 

 fuJly authorifed to conclude, that there muft be fome ftrong 

 refemblance between the nature of found and that of light. 

 There is pro- I have not, in the courfe of thefe inveftigations, found 



^n'-'Ledii^^'^" ^"^ reafon to fuppofe the prefence of fuch an infleding me- 

 dium in the neighbourhood of denfe fubftances as I was for- 

 merly incliped to attribute to them; and, upon copfidering 

 the phenomena- of the aberration of the fi:ars, I am difpofed 

 to believe, that the luminiferous ether pervades the fubr 

 ilance of all material bodies with little or no refi fiance, as 

 freely perhaps as the wind palTes through a grove of trees, 

 Praftical appli- The obfervations on the effeds of ditfraciion and inter- 

 cation ot" the ference, may perhaps fometimes be applied to a pradical 

 purpofe, in majcing us cautious in our copclufions relped^ing 

 the appearances of minute bodies viewed in a microfcope. 

 The fliadow of a fibre, however opaque, placed in a pencil 

 of light admitted through a fmall aperture, is always fome- 

 what lefs dark in the middle of its breadth than in the parts 

 on each fide. A fimilar efied may alfo take place, in fome 

 ' degree, with refped to the image on the retina, and impreis 



the fenfe with, an idea of a tranfparency whi^h has no real 

 exiftence : and, if a fmall portion of li^ht be really tranf- 

 mitted through the fubllance, this may again be deftroyed 

 by its interference with the diflraded light, and produce an 

 appear&nce of partial opacity, inftead of uniform femitranf-* 

 3 parenty. 



