2 5 
the Theory of Light and Colours. 
impulse must then have travelled from an internal to an exter- 
nal circle in an oblique direction, in the same time as in the 
direction of the radius, and consequently with a greater velo- 
city; against the first proposition. In the case of water,, the 
velocity is by no means so rigidly limited as in that of an 
elastic medium. Yet it is not necessary to suppose, nor is it 
indeed probable, that there is absolutely not the least lateral 
communication of the force of the undulation, but that, in highly 
elastic mediums, this communication is almost insensible. In 
the air, if a chord be perfectly insulated, so as to propagate 
exactly such vibrations as have been described, they will in 
fact be much less forcible than if the chord be placed in 
the neighbourhood of a sounding board, and probably in some 
measure because of this lateral communication of motions of an 
opposite tendency. And the different intensity of different parts 
of the same circular undulation may be observed, by holding a 
common tuning fork at arm's length, while sounding, and 
turning it, from a plane directed to the ear, into a position per- 
pendicular to that plane. 
PROPOSITION IIIv 
A Portion of a spherical Undulation , admitted through an Aper- 
ture into a quiescent Medium , will proceed to be further propa- 
gated rectilinearly in concentric Superficies , terminated laterally 
by weak and irregular Portions of newly diverging Undula- 
tions . 
At the instant of admission, the circumference of each of the 
undulations may be supposed to generate a partial undulation, 
filling up the nascent angle between the radii and the surface 
terminating the medium ; but no sensible addition will be made 
MDCCCII, E 
