6i2 



EXPERIMENTS AND CALCULATIONS 



Exper. 4. 



Breadth of the wire 



Distance of the wire from the aperture 



Distance of the wall from the aperture 



(Breadth of the shadow, by three measurements 



Distance of the first pair of darlv hues 



Tnterval of disappearance 



Distance of the secontl pair of dark lines 



Interval of di:^appearance 



Distance of the third pair of dark lines 



Interval of disappearance .... 



.083. 



.32. 



250. 



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



1.165, 1.170, or 1.160; mean, 1.165. 



.0000194. 

 1.402, 1.395, or 1.400 ; mean, 1.399- 



.0000137. 



1.594, 1.580, or 1.585 ; mean, 1.586. 



. \ . ... , .0000128. 



It appears, from five of the six observations 

 of the first Table, in which the distance of 

 the shadow was varied from about 3 inches 

 to 1 1 feet, and the breadth of the fringes 

 was increased in the ratio of 7 to 1, that the 

 diiference of the routes, constituting the in- 

 terval of disappearance, varied but one 

 eleventh at most ; and that, in three out of 

 the five, it agreed with the mean, either ex- 

 actly, or within t4o part. Hence we are 

 warranted in inferring, that the interval, ap- 

 propriate to the extinction of the brightest 

 light, is either accurately or very nearly con- 

 stant. 



But it may be inferred, from a compari- 

 son of all the other observations, that when 

 the obliquity of the reflection is very great, 

 some circumstance takes place, which causes 

 the interval thus calculated to be somewhat 

 greater: thus, in the eleventh line of the 

 third Table, it comes out one sixth greater 

 than the mean of the five already mentioned. 

 On the other hand, the mean of two of 

 Newton's experiments and one of mine, is a 

 result about one fourth less than the former. 

 With respect to the nature of this circum- 

 stance, I cannot at present form a decided 



opinion ; but I conjecture that it is a devia- 

 tion of some of the light concerned, from the 

 rectilinear direction assigned to it, arising 

 either from its natural diffraction, by which 

 the magnitude of the shadow is also enlarged, 

 or from some other unknown cause. If we 

 imagined the shadow of tbp wire, and the 

 fringes nearest it, to be so contracted, that 

 the motion of the light bounding the sha- 

 dow might be rectilinear, we should thus 

 make a sufficient compensation for this de- 

 viation ; but it. is difficult to point out what 

 precise track of the light would cause it to 

 require this correction. 



The mean of thfc three experiments, which 

 appear to have been least affected by this un- 

 known deviation, gives .0000127 for the in- 

 terval appropriate to the disappearance of 

 the brightest light ; and it may be inferred, 

 that if they had been wholly exempted from 

 its effects, the measure would have been 

 somewhat smaller. Now the analogous in- 

 terval, deduced from the experiments of 

 Newton on thin plates, is .00001 12, which is 

 about one eighth less than the former result ; 

 and this appears to be a coincidence fully 

 sufficient to authorise us to attribute these 



