43 
the Theory of Light and Colours. 
route, will so interfere with it, as to cause an appearance of 
colours. The length of the two tracks will differ the less, as 
the direction of the reflected light has been less changed by its 
reflection, that is, in the light passing nearest to the margin ; so 
that the blues will appear in the light nearest the shadow. The 
effect will be increased and modified, when the reflected light 
falls within the influence of the opposite edge, so as to interfere 
with the light simply inflected by that also. 
But, in order to examine the consequences more minutely, it 
will be convenient to suppose the inflection caused by an ethereal 
atmosphere, of a density varying as a given power of the dis- 
tance from a centre, as in the eighth proposition of the last 
Bakerian Lecture. (Phil, Trans, for 1801, p. 83.) Putting 
r = 3, and x =f, I have constructed a diagram, (Fig. 4,) which 
shows, by the two pairs of curves, the relative position of the re- 
flected and unreflected portions of any one undulation at two 
successive times, and also, by shaded lines drawn across, the parts 
where the intervals of retardation are in arithmetical progression,, 
and where similar colours will be exhibited at different distances 
from the inflecting substance. The result fully agrees with the 
observations of Newton’s third book, and with those of later 
writers. But I do not consider it as quite certain, until further 
experiments have been made on the inflecting power of dif- 
ferent substances, that Dr. Hooke’s explanation of inflection, 
by the tendency of light to diverge, may not have some preten- 
sions to truth . I am sorry to be obliged to recall here the assent 
which, at first sight, I was induced to give to a supposed im- 
provement of a late author. (PhiL Trans, for 1800, p. 128.) 
Scholium. In the construction of the diagram, it becomes ne- 
cessary to find the time spent by each ray in its passage, 
G 2 
