2832 Dr. Herschel's Experiments for investigating 
This experiment is so simple, and points out the general 
causes of the rings which are here produced in so plain a 
manner, that we may confidently say they arise from the 
flection of the rays of light on the particles of the floating 
powder, modified by the curvature of the reflecting surface 
of the mirror. 
Here we have no interposed plate of glass of a given thick- 
ness between one surface and another, that might produce the 
colours by reflecting some rays of light and transmitting 
others ; and if we were inclined to look upon the distance of 
the particles of the floating powder from the mirror as plates 
of air, it would not be possible to assign any certain thickness 
to them, since these particles may be spread in the beam of 
light over a considerable space, and perhaps none of them will 
be exactly at the same distance from the mirror. 
I shall not enter into a further analysis of this experiment, 
as the only purpose for which it is given in this place is to 
show that the principle of thin or thick plates, either of air or 
glass, on which the rays might alternately exert their fits of 
easy reflection and easy transmission, must be given up, and 
that the fits themselves of course cannot be shown to have 
any existence. 
XXXIV. Conclusion . 
It will hardly be necessary to say, that all the theory relating 
to the size of the parts of natural bodies and their interstices, 
which Sir I. Newton has founded upon the existence of fits 
of easy reflection and easy transmission, exerted differently, 
according to the different thickness of the thin plates of which 
he supposes the parts of natural bodies to consist, will remain 
unsupported ; for if the above mentioned fits have no existence, 
