OK THE THSOIIT OI' tieHT AN» COtOCBS. 



6l^ 



that the vavioiw colours of white %ht should ture ; on the contrai'y, in 4 circular wave of 



water, every part is, usually, at the saine in- 

 sUmt either elevated or depressed. It may 

 be difficult to show mathematically the 

 mode, in which this intquahty of force is pre- 

 served: but the inlerence from the matter of 

 fact ajipears to be unavoidable. The theory 

 of Huygens indeed explains the cireitin- 

 stance in a manner tolerably satisfactory ; 



intermix their undulations; for, supposnig 

 the vibrations of tlie retina to continue but a 

 five hundredth of a second after their excite- 

 ment, a m^illion uadula^itms of each of a 

 million colours B»ay arrive, iu distinct succes- 

 sion, within this inteival of time, and produce 

 the same sensiole effect, as if ail the colours 

 arrived precisely at the same instant. 



Proposition ii. Jn Uudnlation, conceived he supposes every particle of the medium to' 



to originate from the libralton of a single propagate a distinct undulation in all direc- 



JPurticle, must erpaitd through a homogeneous tions ; and that the general effect is only 



Medium in a spherieal Form, hut zeith dif- perceptible where a portion of each undula- 



ferent Quantities of Motionin different Parts. tion conspires in direction at the same in- 



For, since every imijulse, considered as slant ; and it is easy to show that sucli a ge- 



potiiiive or negative, is propagated with a neral undulation, would, in all cases, proceed' 



constant velocity, each part of the undula- rectilineavly, with proportionate force ; but, 



tion must, in equal times, have past through upon this supposition, it seems to follow, 



equal distance-i from the vibrating point, that a greater quantity of force must be lost 



And, sui)posing the vibrating particle, in the by the divergence of the partial undulations, 



course of its motion, to proceed forwards to than apj)ears to be consistent with the pro- 



a small distance in "a given direction, the pagation of the effect to any considerable 



principal strength of the undulation will na- distance. Yet it is obvious, that some such 



turally be straigh' before it ; behind it, the limitation of the motion must naturally be 



motion will be equal, in a contrary direction; expected to take place ; for, if the intensity 



and, at right angles to the line of vibration, of the motion of any pai'ticular part, instead 



the undulation will be evanescent. of continuing to be propagated straight for- 



Now, in order that such an undulation wards, were supposed to affect the intensity 



may continue its progress to any consider- of a neighbouring part of the undulation, an 



able distance, there must be, in each part of impulse must then have travelled from an in- 



it, a tentlency to preserve Us own motion in ternalto an external circle, in an oblique di- 



a right line from the centre ; for, if the ex- 

 cess of force at any part were communicated 

 to the neighbouring particles, there can be 

 no reason why it should not very soon be 

 equalised throughout, or, in other words, be- 

 come wholly extinct, since the motions in 

 contrary directions would naturally destroy 

 each other. The origin of sound from the 

 ribratioa of a chord is evidently of this na- 



rection, in the same time as in the direction 

 of the radius, and consequently with a 

 greater velocity ; against the first proposition. 

 In the case of water, the velocity is by no 

 means so rigidly limited as in that of an elas- 

 tic medium. Yet it is not necessary to sup- 

 pose, nor will the phenomena of light even 

 allow us to admit, that there is absolutely not 

 the least lateral communication of the force of 



