364 LECTURE XXXIX. 



effects of waves of different breadths. The simple calculation of the velo- 

 city of waves, propagated in a liquid perfectly elastic, or incompressible, 

 and free from friction, assigns to them all precisely the same velocity, what- 

 ever their breadth may be : the compressibility of the fluids actually exist- 

 ing introduces, however, a necessity for a correction according to the 

 breadth of the wave, and it is very easy to observe, in a river or a pond of 

 considerable depth, that the wider waves proceed much more rapidly than 

 the narrower. We may, therefore, consider the pure ethereal medium as 

 analogous to an infinitely elastic fluid, in which undulations of all kinds 

 move with equal velocity, and material transparent substances, o\\ the 

 contrary, as resembling those fluids, in which we see the large waves ad- 

 vance beyond the smaller ; and by supposing the red li^t t& consist of 

 larger or wider undulations and the violet of smaller, we may sufficiently 

 elucidate the greater refrangibility of the red than of the violet light.* 



It is not, however, merely on the ground of this analogy that we may be 

 induced to suppose the undulations constituting red light to be larger than 

 those of violet light : a very extensive class of phenomena leads us still more 

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

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

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

 manner sufficiently minute or comprehensive to satisfy the most candid 

 even of the advocates for the projectile system ; while on the other hand 

 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, are heard to vibrate together. 



Supposing the light of any given colour to consist of undulations of a 

 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 the 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 at other points to 

 redouble 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 devi- 

 ating 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 



* See Cauchy, Memoire sur la Dispersion de la Lumiere, Prague, 1835. Powell, 

 Ph. Mag. vi. 16, 107, 189, 262. Ph. TV. 1835, p. 249, &c. ; and Essay on the Un- 

 dulatory Theory, as applied to the Dispersion of Light. Challis, Ph. Mag. viii. 

 Kelland, Trans. Camb. Ph. Soc. vi. 153. Difference of colour was referred to dif- 

 ference of velocity by Melvil, Ph. Tr. 1753, p. 262, and Essays, ii. 12. 



