620 



ON THE THEORT OF LIGHT AND COLOURS. 



the undulation, but it appears that, in highly 

 elastic mediums, this communication is al- 

 most insensible. In the air, if a chord be 

 perfectly insulated, so as to propagate ex- 

 actly 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 communica- 

 tion 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 

 perpendicular to that plane. 



Proposition hi. A Portion of a sphe- 

 rical Undulation, admitted ihrough an Aper- 

 ture into a quiescent Medium, tcill proceed to 

 be further propagated rectilinearly in con- 

 centric Superficies, terminated laterulhj hy 

 weak and irregular Portions of newly diverg- 

 ■ ing Undulations. 



Ax. the instant of admission, the circum- 

 ference of each of the undulations may be 

 supposed to generate a partial unduhilion, 

 filling up the nascent angle between the 

 radii and the surface terminating the me- 

 dium ; but no sensible addition will be made 

 to its strength by a divergence of mo- 

 tion from any other parts of the undulation, 

 for want of a coincidence in time, as has 

 already' been explained with respect to the 

 various force of a spherical undulation. If 

 indeed the aperture bear but a small propor- 

 tion to the breadth of an undulation, the 

 uewly generated undulation may nearly ab- 

 sorb the whole force of the portion admitted; 

 and this is the case considered by Newton in 

 the Principia. When an experiment is made 



under these circumstances in light, it is cer- 

 tain that, whatever may be the cause, it by no 

 means wholly retains a rectilinear direction. 

 Let the concentric lines in Fig. 105. .(Plate 

 14.) represent the contemporaneous situa- 

 tion of similar parts of a number of succes- 

 sive undulations diverging from the point 

 A ; they will also represent the successive 

 situations of each individual undulation : 

 let the force of each undulation be repre- 

 sented by the breadth of the line, and let the 

 cone of light ABC be admitted through the 

 aperture BC ; then the principal undulations 

 will proceed in a rectilinear direction towards 

 Gil, and the faint radiations on each side 

 will diverge from B and C as centres, with- 

 out receiving any additional force from any 

 intermediate point D of the undulation, on 

 account of the inequality of the lines DE and 

 OF. But, if we allow some little lateral 

 divergence from the extremities of the undu- 

 lations, it must diminish their force, with- 

 out adding materially to that of the dissi- 

 pated light ; and their termination, instead 

 of the right line BG, will assume the form 

 CH ; since the loss of force must be more 

 considerable near to C than at greater dis- 

 tances. This line corresponds with the 

 boundary of the shadow in Newton's first ob- 

 servation. Fig. 1 ; and it is much more pro- 

 bable that such a dissipation of light was the 

 cause of the increase of the shadow in that 

 observation, than that it was owing to the 

 action of an inflecting atmosphere, or of an 

 attractive force, which must have extended 

 a thirtieth of an inch each way in order to 

 produce it; especially when it is considered 

 that the shadow was not diminished by sur- 

 rounding the hair with a denser medium 

 than air, which must in all probability have 



