Mr EARNSHAW, ON FLUID MOTION. 233 



Secondly, That if such a wave were excited, it could not be trans- 

 mitted in that form. 



Hence, if a curvilinear wave in traversing a medium meet with a 

 fixed screen in which is an orifice, the part of the wave which passes 

 through the orifice must afterwards abut with its edges upon the back 

 of the screen. 



35. In the Undulatory Theory of Light, each point in the front of 

 a wave is considered as the origin of an indefinitely small wave (See 

 Airy's Tracts, 2d Edit. p. 267. Art. 21). This hypothesis, however, as 

 IS well known, is afFected by a very troublesome difficulty. " What is 

 to limit the waves diverging from each of these small sources of mo- 

 tion ? The disturbance spreads generally in a spherical form, so that 

 the front of each little wave is a sphere: are we to suppose the sphere 

 complete, so that each small undulation is propagated backwards as well 

 as forwards?" (Airy, Art. 22.) 



It wiU have been perceived from what has been done in this paper, 

 that in the transmission of waves by pressure through an elastic me- 

 dium, the tangent-planes are to be taken, and that these tangent-planes 

 move only in the direction of the wave's motion. Might not the same 

 hypothesis be applied in the Undulatory Theory of Light, in which 

 case the above difficulty would be avoided? 



