202 
DR. BREWSTER ON REFLECTED LIGHT. 
law in solids and fluids. There seems to be no method of determining 1 whether 
or not this is the case ; for experiment indicates only the total effect, or the sum 
of all the ordinates, and these may be compensated, though they vary according 
to different laws. 
There is one hypothesis, however, on which the preceding experiments may 
be reconciled with the supposition of the mutual dependence of the reflecting 
and refracting forces. If we suppose, for example, as in Fig. 3, that the refract- 
ing forces of the solid and fluid are regulated by the same curves as their 
reflecting forces, and that the absolute effect of each is the same ; then, 
though the refractive forces are perfectly balanced, and though the total effect 
of eacli reflecting force taken separately is the same in the solid as in the fluid, 
yet light will still be reflected in the manner formerly described. It seems 
highly probable that the law of the refracting force varies in different bodies ; 
and if we take for granted the mutual dependence of the refracting and reflect- 
ing forces, the preceding experiments will establish a variation in the law of 
the refracting forces of different media. 
In the undulatory system, the preceding facts may be explained by sup- 
posing that the density or elasticity of the ether varies near the surface of dif- 
ferent bodies ; a supposition in itself highly probable, and which has been 
already adopted to explain the loss of part of an undulation in several of the 
phenomena of interference. In such a case the reflection of the light will 
commence at a line where the density or elasticity of the ether in the first 
medium begins to change, and will continue till the ray has penetrated to that 
part of the second medium where the density or elasticity of the ether is uni- 
form. In this theory, therefore, the preceding facts may be regarded as prov- 
ing the variable condition of the ether near the surfaces of bodies, and of 
establishing the beautiful and sagacious deduction of Dr. Young, that the part 
of an undulation lost is a variable fraction depending on the nature of the 
contiguous media. 
II. We come now to consider the second class of phenomena, or the ex- 
istence of periodical colours at the confines of certain media of the same and 
of different refractive powers. 
That the periods of colour arise, as in all similar phenomena, from the inter- 
