PHYSICAL OPTICS, 



133 



Exper. 4. 



Breadth of the wire - , - - - ,083« 



Difiance of the wire from the aperture ... 32. 



Diflanceof the wall from the aperture - « - 250. 

 (Breadth of the (hadow by three 



meafureraents - - .815, .826, or .827; mean, .823.) 



DiHance of the firft pair of dark 



lines - . - - 1.165, 1.170, or 1.160; mean, 1.165. 



Interval of difappearance - . - - .0000194. 



Diftanceof the fecond pair of dark 



lines - - - . 1.402, 1.395, or 1.400; mean, 1.399. 



Interval of difappearance - - - - .0000137. 



Diftance of the third pair of dark 



Hnes .... 1.594, 1.580, or 1.585; mean, 1,5&6. 



Interval of difappearance _ - - . .0000128. 



It appears, from five of the fix obfervallons of the firft Table, 

 in which the diftance of the (hadow was varied from about 3 

 inches to 1 1 feet, and the breadth of the fringes was increafed 

 ill the ratio of 7 to I, that the difference of the routes confli» 

 tuting the interval of difappearance, varied but one-eleventh 

 at mofi; and that, in three out of the five, it agreed with the 

 mean, either exactly, or within ^^ part. Hence we are war- 

 ranted in inferring, that the interval appropriate to theextinftion 

 of the brightefl light, is either accurately or very nearly 

 conflanf. 



But it may be inferred, from a comparlfon of all the other 

 obfervations, that when the obliquity of the refledlion is very 

 great, fome circumflance takes place, which caufes the interval 

 thus calculated to be fomewhat greater ; thus, in the eleventh 

 line of the third Table, it comes out one-fixth greater than the 

 mean of the live already mentioned. On the other hand, the 

 mean of two of Newton's experiments and one of mine, is a 

 refult about one-fourth lefs than the former. With refpedl to 

 the nature of this circumfiance, I cannot at prefent form a de- 

 cided opinion; but Iconje61ure thit it is a deviation of fome of 

 the light concerned, from the redilinear direftionalligned to it, 

 arifing either from its natural diffraclion, by which the mag- 

 nitude of the fiiadow is alfo enlarged, or from fome other un- 

 known caufe. If we imagined the fliadow of the wire, and the 

 5k fringes 



