the Theory of Light and Colours. 35 
more or less perfectly in the times of performing their respective 
motions. 
Corollary i. Of the Colours of striated Surfaces. 
Boyle appears to have been the first that observed the colours 
of scratches on polished surfaces. Newton has not noticed them. 
Mazeas and Mr. Brougham have made some experiments on 
the subject, yet without deriving any satisfactory conclusion. But 
all the varieties of these colours are very easily deduced from 
this proposition. 
Let there be in a given plane two reflecting points very near 
each other, and let the plane be so situated that the reflected 
image of a luminous object seen in it may appear to coincide 
with the points ; then it is obvious that the length of the inci- 
dent and reflected ray, taken together, is equal with respect to 
both points, considering them as capable of reflecting in all 
directions. Let one of the points be now depressed below the 
given plane; then the whole path of the light reflected from it, 
will be lengthened by a line which is to the depression of the 
point as twice the cosine of incidence to the radius. Fig. 2. 
If, therefore, equal undulations of given dimensions be reflected 
from two points, situated near enough to appear to the eye but 
as one, wherever this line is equal to half the breadth of a whole 
undulation, the reflection from the depressed point will so in- 
terfere with the reflection from the fixed point/ that the pro- 
gressive motion of the one will coincide with the retrograde 
motion of the other, and they will both be destroyed ; but, when 
this line is equal to the whole breadth of an undulation, the 
effect will be doubled ; and when to a breadth and a half, again 
destroyed ; and thus for a considerable number of alternations ; 
and, if the reflected undulations be of different kinds, they will 
F 2 
