( $o8S ) 
Igithe fecond pkcej I told you^that the Ob'j^ctotsHjpothejlf^ 
, asto thefandamcntal partof it, is 
t^^^f::::^^'':^ not ag^ina me. That fundamental 
Bitumnrf that and all othr Mecha- Suppolition is ; Thai: the parts of 
tMrhe^''^''^'' «r^»^rm^^/.ioi«;' y^^.^^^ ^^^^^/^ agitated, do ex- 
cite Vibrations in the Mther^ which are 
propagated every way from thofe bodies in \ir eight lines ^ and caufe a 
iSenjation oj Light by beating and dajhing^ agawfi th bottorm of the 
Eye^ (ometbmg after the manner that Vibratims in the Air caufe a 
Senfation of Sound by beating againfl the Organs of Hearing. Now, 
the moft free and natfural ApplicaiioD of this Hypothtjis to the 
Soi\xi\on of phienomena I take to be this ; That the agitated parts 
of bodiesj according to their feveral fizesj figures, and mos 
tionsv do excite Vibrations in the atb^r of various depths or 
bignefleSj^which being promifcuoufly propagated through that 
Msdium to our Eyes^eiSecfl in us a Seofation of Light ot a White 
colour; butif by any means thofe of unequal bignefles be fe-- 
parated from one another, the largeft beget a Senfotion of a 
-S^^colour, theleaft or fliorteft, of ^ deep Violet^ and t lie in- 
terraediatones, of intermediat colors ; much after the man- 
ner that bodies, according to their feveral fizes, fliapes, and 
f motions^ excite vibrations in the Air of various bigneffes , 
which, according to thofe bigneflTes, make feveral Tones in 
Sound : That the largeft Vibrations are befl: able to over- 
come the refiftacce of a Refradiog fuperficies^ and fo break 
through it with leaft Refradion 5 whence the Vibrations of 
feveral bigneffes, that is, the Rays of feveral Colors, which 
are blended together in Light, muft be parted from one ano- 
ther by Refraition, and fo caufe the Phtsnomena of Pr?fmes and 
other refrafting fubftance§ : Atid that it depends on the thick- 
nefs of a thin tranfparent Plate or Buble, whether a Vibration 
ftiall be refleUed at its further fuperficies,or tranfmitted i fo that, 
according to the number of vibrations, interceding the two 
fupcrficies,they may be refledied or tranfmitted for many fuc- 
ceflive thicknefTes. And fince the Vibrations which make 
B/^tt? and Violet^ are fuppofcd fliorter than thofe which make 
Red and Tellorvy they muftberefleftedat a lefs thickncfs of the 
Plate : Which is fufficient to explicate all the ordinary fhanos 
mena ofihoit Plates or Bubles, and alio of all natural bodies, 
whofe 
