10 On the Diff'raction of an Annular Aperture. 



Suppose, for instance, a circular plate 1 foot in diameter is 

 placed to interrupt a stream of light ; a screen is placed 10 

 feet behind it ; to find the diameter of the bright spot. 



Here h — \20 inches, a = 6 inches; \ may be taken for 

 mean light = 0-00002 inch. Substituting in the expression 

 above, we find the diameter of the bright spot =0*000308 inch, 

 or less than jJ^jy inch, regarding the visibility of which in 

 common experience we need not to disquiet ourselves. 



If the diameter of the circular plate had been taken 1 inch, 

 the distance of the screen remaining the same, the diameter 

 of the spot would have been 0*0037 inch, a speck difficult even 

 for a philosopher to discover under these circumstances. If 

 the diameter of the plate were O'l inch, the diameter of the 

 spot would be 0-037 inch, a very fit subject for experimental 

 measure. It is on such spots as this that observations have 

 been made, with the purpose of testing the undulatory theory, 

 and of the agreement of those measures with the theory no 

 one acquainted with both has, I believe, doubted. 



With regard to any objection that may have been made, 

 either against the undulatory theory generally or against the 

 application of Huyghens's principle in particular, from con- 

 fronting the result of a simple investigation of the intensity of 

 light at one mathematical point, with the fact of general ex- 

 perience that light is not visible behind an opake screen : the 

 inference tiom the preceding investigation is, I believe, that 

 the undulatory theory in general, and the application of 

 Huyghens's principle in particular, stand as firmly as they did 

 before appeal was made to this comparison, perhaps even more 

 firml}'. For the future I would I'emark that, however much 

 we may doubt whether one mathematician is entitled to give 

 to another a lesson oi catition, this principle must, I think, be 

 allowed by every one, — that a theory standing on such varied 

 and such extensive evidence as that which supports the un- 

 dulatory theory is entitled to the same respect which is given 

 to a private person of high character ; it is not to be rashly 

 attacked until the supposed grounds of attack have been 

 most thoroughly examined, although if these grounds are 

 valid, it possesses no immunities beyond other theories. 

 I am, Gentlemen, 



Your very obedient servant. 

 Royal Observatory, Greenwich, Dec. 4, 1840. G. B. AlRY. 



