178 Mr. Barton on the Infection of Light. 



excepting a small orifice or slit, and blow strongly through 

 it against the flame of a candle. The flickering motion of the 

 flame will show that the air does not issue from the orifice in 

 an uniform stream, but in a succession of rapidly alternating 

 movements. This fact was observed by Dr. Young, and de- 

 scribed by him in a paper published in the Philosophical 

 Transactions for 1800. The mode of observation that he 

 employed was, indeed, somewhat different from that here de- 

 scribed : he contrived to render the issuing current of air vi- 

 sible by combining it with smoke. If the pressure is increas- 

 ed, a whistling sound is produced; affording another kind of 

 proof that the air in passing through the orifice is thrown into 

 a state of undulation. 



Now, if the particles of light be supposed to constitute an 

 elastic fluid, it is evident that a phenomenon of the same 

 kind must take place whenever a stream of this fluid impinges 

 upon a narrow aperture; and in this case the formulae of 

 Fresnel will be applicable, with some modification, to the hy- 

 pothesis here proposed, as well as to the hypothesis which 

 supposes light to consist in the vibrations of an imaginary 

 aether; and with this advantage, that an obvious reason is 

 assigned why the undulations are found to have their origin 

 at the aperture by which the light enters : for which no rea- 

 son, as it appears to me, can be given on the opposite theory. 



By the help of the same principle it might also be ex- 

 plained, I think, why a beam of light passing through a very 

 narrow slit, is separated into two portions, leaving a dark 

 space between them ; and why the formulae of Fresnel are 

 found to fail in approaching this limit. But it would, per- 

 haps, be out of place at present to enlarge on these questions, 

 which are not likely to excite much interest while the theory 

 on which they rest is not yet recognised or established. I 

 could bring forward other considerations in abundance which 

 appear to me decisive against the undulatory hypothesis ; but 

 it seems preferable thoroughly to investigate and discuss, in 

 the first instance, the circumstances of one or two classes of 

 phaenomena, rather than to lose ourselves in a wider circle 

 of reasoning. 



I cannot conclude without again expressing my sense of the 

 friendly spirit in which Professor Powell has criticized my 

 former observations. I should be fastidious indeed, if I did 

 not perceive and acknowledge that his remarks deserve to be 

 considered in no other light than as intended, and as calcu- 

 lated, " to promote the common cause of scientific truth." 



