294 On changeable Colours and Glories, 
supposed on the principles of the undulatory theory. But 
when the obliquity is so considerable, it is not very easy to 
assign a reason for this inversion; and the experiments, 
which I have now mentioned, make it necessary to assume 
a law, which I cannot explain, that every very oblique re- 
flection inverts the properties of light with respect to inter- 
ference. This conclusion confirms the assertion of Newton, 
that a dark space, bordered by light, will appear in the 
cenire of a portion of light transmitted between the edges 
of two knives placed very near each other, and the opinion 
of Mr. Jordan, that the augmentation of a shadow by dif- 
fraction is to be considered as the first dark space helonging 
to the coloured fringes. I had obtained a different result in 
an experiment similar to Newton’s, because I was not aware 
of the necessity of employing very sharp edges; for, when 
the edges are blunt, the light is reflected from the one to the 
other in such a manner, as wholly to destroy the appearance 
of a central dark space; but in any case this source of error 
may be avoided, by causing one of the edges to advance a 
very little before the plane of the other, so that half of the 
fringes may disappear. It is however necessary to suppose 
this inversion confined to cases of extremely oblique re- 
flection ; for, when the deviation of the light from a recti- 
linear path becomes a little more considerable, its effects 
are no longer perceptible; the second and third fringes 
scarcely ever requiring any material corrections of the cal- 
culations from which it is excluded. The same inversion 
must also be attributed to the light bent by diffraction 
round the remoter side of a fibre: fer this light always co- 
Operates in the first instance with that which is reflected 
from the nearer side. The extent of the central white light 
is indeed so great, that all the coloured appearances may 
almost be considered as beginning at such a distance, that 
the first dark space is exactly where the simple calculation 
would Jead us to expect the white; since the value of the 
unit of the eriometer ought to be, according to this calcu- 
Jation, about 75... of an inch, instead of +5453 and in- 
deed this value agrees very accurately with experiment, 
where the two portions of light concerned are exactly in 
similar circumstances ; as may be observed in some of the 
parallel lines drawn on glass in Mr. Coventry’s micrometers, 
probably where they happen to be single, for in general 
they are double, and exhibit colours corresponding to an 
interval much smaller than their regular distance: but in 
some parts we may observe colours exactly corresponding 
to their distance, for instance, to +4, of an inch, according 
to 
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——=S—ee 
