the Inflection , Reflection , and Colours of Light. 245 
and unchangeable ; that they differ in flexibility and refrangi- 
bility ; that they bear the same part in forming images by 
reflection, and fringes by flexion, and colours from thin plates, 
which the rays separated by the prism do : and in the 8th 
experiment we see, that when the rays are placed in the same 
situation with respect to refraction, whether out of a rarer into 
a denser or a denser into a rarer medium, in which they be- 
fore were with respect to reflection, the position of the colours 
produced is diametrically opposite in the tw v o cases. Seeing 
then that in all sorts of light, direct, refracted, reflected, simple, 
and homogeneous, or heterogeneous and compounded, and in 
whatever way the separation and mixture may have been made, 
some of the rays at equal or the same incidences are con- 
stantly reflected nearer the perpendicular than the mean rays, 
and others not so near; and seeing that by such reflection the 
compound ray, of whatever kind, is separated into parts so 
simple that they can never more be changed ; and considering 
the different places to which these parts are reflected ; it is evi- 
dent, that the sun’s li^ht consists of parts different in reflexi- 
bility, and that those which are least refrangible are most re- 
flexible. By reflexibility, I here mean a disposition to be re- 
flected near to the perpendicular in any degree. 
Although I have given what I take to be sufficient proof of 
this property of light, yet I am aware that something more 
is requisite. It will be asked, why does neither a plain, a 
common convex, nor a common concave mirror separate the 
rays by reflection ? This is what has always hindered us from 
even suspecting such a thing as different reflexibility. I shall, 
however, take an opportunity of removing this obstacle, in the 
second part of the plan, when I come to explain the reason of 
