REPORT ON PHYSICAL OPTICS. 21 



It is a union of unconnected principles, which can at best be con- 

 sidered but as supplying the materials for a higher generalization. 

 Its very complexity furnishes a presumption against its truth ; for 

 the higher we are permitted to ascend in the scale of physical in- 

 duction, the more we perceive of that harmony, and unity, and 

 order, which must reign in the works of One Supreme Author. 

 The theory of emission, in its present state, exhibits all these 

 symptoms of unsoundness ; but there is something stronger than 

 mere presumption against it. It will appear, I think, upon a fair 

 review, that in almost every instance in which it has been de- 

 veloped, its consequences are at variance with facts ; and the proof 

 of its insufficiency seems even stronger than the positive evidence 

 in favour of the rival theory. 



In proceeding to the consideration of these arguments, I have 

 found it necessary to deviate from the arrangement which a strictly 

 theoretical view of the subject would naturally suggest. The re- 

 lation 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 following Eeport 

 into two parts, of which the first will treat of unpolarized, and the 

 second of polarized light. In the former I shall consider sepa- 

 rately 



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 mira- 



tions ; 



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 



