REPORT ON PHYSICAL OPTICS. 297 



variance with facts ; and the proof of its insufficiency seems 

 even stronger than tlie positive evidence in favour of the rival 

 theory. 



In proceeding to the consideration of these arguments, I have 

 foiindit necessary to deviate from tlie arrangement which a strictly 

 theoretical view of the subject would naturally suggest. The 

 relation of theory to phenomena, which I propose to consider, 

 obliges me to examine the latter in the groups in which they 

 have been usually brought together, and under which their laws 

 have been investigated. I propose, therefore, to divide the fol- 

 lowing Report into two parts ; of which the first will treat of 

 nnpolarized, and the second oi polarized, light. In the former 

 I shall consider separately, 



1 . The propagation of light, and the principle of interference ; 



2. The reflexion and refraction of light; 



3. Diffraction; 



4. The colours of thin and thick plates. 



The second part will comprise, 



1. The polarization of light, and the principle of transversal 



vibrations ; 



2. The reflexion and refraction of polarized light ; 



3. Double refraction ; 



4. The colours of crystallized plates. 



Many subjects of high interest are omitted in this arrangement, 

 as being but remotely connected with the leading object of the 

 present Report. I have left wholly untouched, for this reason, 

 that branch of optical science which is sometimes denominated 

 " mathematical optics," or the development of the fundamental 

 laws of reflexion and refraction. The phenomena of vision 

 have been in like manner omitted, as involving also the science 

 of physiology ; and the relations of light to other agents, as 

 heat, electricity, and magnetism, because these relations are as 

 yet little understood, and in the present state of the kindred 

 sciences, the science of light can hope to derive little aid from 

 their examination. These interesting subjects would, each of 

 them, well merit a separate consideration. 



Part I. Unpolarized Light. 



I. Propagation of Light. Principle of Interference. 



The first property of light which claims our notice is its pro- 

 gressive movement. Light, we know, travels from one point 



