ON LIGHT. 335 



(ii6.) Take now the case of the exterior fringes, when 

 the shadow of a broad straight-edged body, as a ruler, is 

 tlirown on a fixed screen at a considerable distance be- 

 hinc it. Suppose p first placed exactly at the edge of 

 the geometrical shadow. In that case, the view of ex- 

 actly half of each of the concentric wave-zones (a), (i;), 

 (c), &c., will be intercepted, and p will therefore receive 

 from the remaining halves just half the amount of luniin- 

 ifcroiis agitation it received when opposed to tlie wliole 

 wave, viz., half the amount of concordant and half of 

 discordant undulation. Its intensity oi illuniinatioti will 

 therefore be one-foicrth of timt when the ruler is altogether 

 removed.* Now, suppose the ruler withdrawn gradually, 

 and laterallj, so as to disclose to the view of p succes- 

 sively, ist, the whole of the central zone (a) of the wave 

 surface; 2dly, the whole of the two first zones (a), (b) ; 

 3dly, the three first, (a), (b), (c), and so on. It is very 

 evident then, on merely casting our eyes on Fig. 6, (p. 

 295), and imagining a line drawn through the common 

 centre of all the circles to be removed parallel to itself, 

 step by step, so as to become in succession a tangent to 

 the ist, 2d, 3d, &c., circles; that in the first step of its 

 removal it will disclose to p all the remaining half of the 

 central area (a), which sends to it undulations concordant 

 with those by which ? is already illuminated, but less 



* The eficct on the retina is estimated, not by tlie simple moiiwii- 

 tiim ox velocity o^ the impulse communicated by tlie vibration, but 

 by the "w'j viva," "energy," or "work done," which is proportion- 

 ate to the square of the velocity of movement. In this the undulatory 

 doctrme of light agrees with the theory of sound. 



