6 Dr. Young’s Experiments and Calculations 
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 difference of the routes constituting 
the interval of disappearance, varied but one-eleventh at most ; 
and that, in three out of the five, it agreed with the mean, either 
exactly, or within T ~ part. Hence we are warranted in in- 
ferring, that the interval appropriate to the extinction of the 
brightest light, is either accurately or very nearly constant. ' 
But it may be inferred, from a comparison 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 circumstance, I cannot at present form a de- 
cided opinion ; but I conjecture that it is a deviation of some of 
the light concerned, from the rectilinear direction assigned to it, 
arising either from its natural diffraction, by which the mag- 
nitude of the shadow is also enlarged, or from some other 
unknown cause. If we imagined the shadow of the wire, and the 
fringes nearest it, to be so contracted that the motion of the 
light bounding the shadow might be rectilinear, we should thus 
make a sufficient compensation for this deviation ; but it is dif- 
ficult to point out what precise track of the light would cause 
it to require this correction. 
The mean of the three experiments which appear to have 
been least affected by this unknown deviation, gives .0000127 
