202 



DR. BREWSTER ON REFLECTED LIGHT* 



law in solids and fluids. There seems to be no method of determining whether 

 or not this is the case ; for experiment indicates only the total effect, or the sxmi 

 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 each 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 

 phaenomena 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 phaenomena, 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 phaenomena, from the inter- 



