88 



Proportion III. 

 Lateral undula- 

 tions explained. 



THtORV OF LIGHT AND COLOURS. 



proportion. In the cafe of water, the velocity is by no 

 means fo rigidly limited as in that of an elaftic medium. Yet 

 it is not neceffary to fuppofe, nor is it indeed probable, that 

 there is abfolutely not the lead lateral communication of the 

 force of the undulation, but that, in highly elaftic mediums, 

 this communication is almott infenfible. In the air, if a 

 chord be perfectly infulated, fo as to propagate exactly fuch 

 vibrations as have been defcribed, they will in fact be much 

 lefs forcible than if the chord be placed in the neighbourhood 

 of a founding board, and probably in fome meafure becaufe of 

 this lateral communication of motions of an oppofite tendency. 

 And the different intenfity of different parts of the fame cir- 

 cular undulation may be obferved, by holding a common 

 tuning fork at arm's length, while founding, and turning it, 

 from a plane directed to the ear, into a pofition perpendicular 

 .to that plane, 



PROPOSITION III. 



A Portion of afpherical Undulation, admitted through an Aper- 

 ture into a quiefcent Medium, willproceed to be further propa~ 

 gated reclilinearly in concentric Superficies, terminated laterally 

 by weak and irregular Portions of newly diverging Uiidula- 

 tions. 



At the inflant of admiffion, the circumference of each of the 

 undulations may be fuppofed to generate a partial undulation, 

 filling up the nafcent angle between the radii and the furface 

 terminating the medium ; but no fenfible addition will be made 

 to its ffrength by a divergence of motion from any other parts 

 of the undulation, for want of a coincidence in time, as has 

 already been explained with refpect to the various force of a 

 fpherical undulation. If indeed the aperture bear but a fmall 

 proportion to the breadth of an undulation, the newly gene- 

 rated undulation may nearly abforb the whole force of the por- 

 tion admitted ; and this is the cafe confidered by Newton in 

 the Principia. But no experiment can be made under thefe 

 circumftances with light, on account of the minutenefs of its 

 undulations, and the interference of inflection ; and yet fome 

 faint radiations do actually diverge beyond any probable hV 

 mits of inflection, rendering the margin of the aperture dif- 

 tinctiy vifible in all directions j thefe are attributed by Newton 

 to fome unknown caufe, diffinct from inflection : (Optics, 



Third 



