454 LECTURE xxxrx. 



more directly to the same conclusion ; they consist chiefly of the production 

 of colours by means of transparent plates, and by diflraction or inflection, 

 none of which have been explained, upon the supposition of emanation, in 

 a manner suiliciently minute or comprehensive to satisfy the most candid 

 even of the advocates for the projectile system; while on the other liand all of 

 them may be at once understood, from the effect of the interference of 

 double lights, in a manner nearly similar to that which constitutes in sound 

 the sensation of a beat, when two strings, forming an imperfect unison, arc 

 lieard to vibrate together. 



Supposing the light of any given colour to consist of undulations, of » 

 given breadth, or of a given frequency, it follows that these undulations 

 must be liable to those effects which we have already examined in the case 

 of the waves of water, and tiie pulses of sound. It has been shown that 

 two equal series of waves, proceeding from centres near each other, may be 

 seen to destroy each other's effects at certain points, and other points at to re- 

 double them; and the beating of two sounds has been explained from a similar 

 interference. We are now to apply the same principles to the alternate union 

 and extinction of colours. (Plate XX. Fig. 267.) 



In order that the effects of two portions of light may be thus combined, 

 it is necessary that they be derived from the same origin, and that they 

 arrive at the same point by different paths, in directions not much deviating 

 from each other. This deviation may be produced in one or both of the 

 portions by diffraction, by reflection, by refraction, or by any of these effects 

 combined ; but the simplest case appears to be, when a beam of homogeneous 

 light falls on a screen in which there are two very small holes or slits, which 

 may be considered as centres of divergence, from whence the light is diffract- 

 ed in every direction. In this case, when the two newly formed beams are 

 received on a surface placed so as to intercept them, their light is divult'd by 

 dark stripes into portions nearly equal, but becoming wider as the surface is 

 more remote ftom the a])ertures, so as to subten : very nearly c^ual angles 

 from the apertures at all distances, and wider also in the same proportion as 

 the apertures art closer to each other. Tlie middle of the two portions is 

 always light, and the bright stripes on each side arc at such distances, that the 

 light, coming to them from one of the ai^rtuies, must have passed through a 



