>4 Dr. Young's Experiments and Calculations 
fringes ; and we shall find, that where the lengths are equal, the 
light always remains white ; but that, where either the brightest 
light, or the light of any given colour, disappears and reappears, 
a first, a second, or a third time, the differences of the lengths 
of the paths of the two portions are in arithmetical progression, 
as nearly as we can expect experiments of this kind to agree 
with each other. I shall compare, in this point of view, the 
measures deduced from several experiments of Newton, and 
from some of my own. 
In the eighth and ninth observations of the third book of 
Newton’s Optics, some experiments are related, which, toge- 
ther with the third observation, will furnish us with the data 
necessary for the calculation. Two knives were placed, with their 
edges meeting at a very acute angle, in a beam of the sun’s 
light, admitted through a small aperture ; and the point of con- 
course of the two first dark lines bordering the shadows of the 
respective knives, was observed at various distances. The results 
of six observations are expressed in the first three lines of the 
first Table, On the supposition that the dark line is produced 
by the first interference of the light reflected from the edges of 
the knives, with the light passing in a straight line between them, 
we may assign, by calculadng the difference of the two paths, 
the interval for the first disappearance of the brightest light, as 
it is expressed in the fourth line. The second Table contains the 
results of a similar calculation, from Newton’s observations on 
the shadow of a hair; and the third, from some experiments of my 
own, of the same nature: the second bright line being supposed to 
correspond to a double interval, the second dark line to a triple 
interval, and the succeeding lines to depend on a continuation 
of the progression. The unit of all the Tables is an inch. 
