Dr , Young’s Lecture on 
3 6 
be variously affected, according to their proportions to the vari- 
ous length of the line which is the difference between the 
lengths of their two paths, and which may be denominated the 
interval of retardation. 
In order that the effect may be the more perceptible, a num- 
ber of pairs of points must be united into two parallel lines ; 
and, if several such pairs of lines be placed near each other, 
they will facilitate the observation. If one of the lines be made 
to revolve round the other as an axis, the depression below the 
given plane will be as the sine of the inclination ; and, while 
the eye and luminous object remain fixed, the difference of the 
length of the paths will vary as this sine. 
The best subjects for the experiment are Mr. Coventry’s 
exquisite micrometers ; such of them as consist of parallel lines 
drawn on glass, at the distance of one five hundredth of an 
inch, are the most convenient. Each of these lines appears 
under a microscope to consist of two or more finer lines, exactly 
parallel, and at the distance of somewhat more than a twentieth 
of that of the adjacent lines. I placed one of these so as to reflect 
the sun’s light at an angle of 45 0 , and fixed it in such a manner, 
that while it revolved round one of the lines as an axis, I could 
measure its angular motion ; and I found, that the brightest red 
colour occurred at the inclinations 10^°, 2of°, 32 0 , and 45 0 ; of 
which the sines are as the numbers 1, 2, 3, and 4. At all other 
angles also, when the sun’s light was reflected from the sur- 
face, the colour vanished with the inclination, and was equal at 
equal inclinations on either side. 
This experiment affords a very strong confirmation of the 
theory. It is impossible to deduce any explanation of it from 
any hypothesis hitherto advanced ; and I believe it would be 
