78 KEPORT ON PHYSICAL OPTICS. 



subdivided state, the portions of light which have traversed them 

 are in a condition to interfere, the interval of retardation depend- 

 ing on the difference of the velocities of light in the two media. 

 Accordingly, coloured rings will be seen when a luminous object 

 is viewed through the glasses; the rings being similar to those 

 usually seen by transmission, but much larger. But when a dark 

 object is behind the glasses, and the incident light somewhat 

 oblique, the rings immediately change their character, and re- 

 semble those of the ordinary reflected system ; one of the portions 

 in this case being reflected, and therefore suffering a loss of half 

 an undulation. These phenomena were observed and explained 

 by Young,* and have been denominated by him the " colours of 

 mixed plates." Young also observed some similar phenomena of 

 colour in an unconfined medium. Thus, when the dust of the 

 lycoperdon is mixed with water, the mixture exhibits a green tint 

 by direct light, and a purple tint when the light is indirect ; and 

 the colours rise in the series when the difference of the refractive 

 densities is lessened by adding salt to the water. The interval of 

 retardation in this case depends also on the magnitude of the 

 transparent particle, f 



In closing the review of this part of the subject, I would 

 observe that any well-imagined theory may be accommodated to 

 phenomena, and seem to explain them, if only we increase the 

 number of its postulates, so as still to embrace each new class of 

 phenomena as it arises. In a certain sense, and to a certain ex- 

 tent, such a theory may be said to be true, so far as it is the mere 

 expression of known laws. But it is no longer a physical theory, 

 whose very essence it is to connect these laws together, and to 

 demonstrate their dependence on some higher principle : it is an 

 aggregate of separate principles, whose mutual relations are un- 

 known. Thus the cycles and epicycles of the Ptolemaic system 

 represented with fidelity the more obvious movements of the plane- 

 tary bodies ; but when the refinements of astronomical research 

 laid bare new laws, new epicycles were added to the system, until 

 at length its complication rendered it useless as a guide. Such 

 appears to be the present state of the theory of emission; and so 



"Account of some Cases of the Production of Colours."-PM. Trans. 1802. 

 Abbe Mazeas noticed many fact 

 . Memoiret presentes, vol. ii. 

 t Introduction to Medical Literature, p. 5,56. 



W*MIWU ui Colours. rim. 



Abbe Mazeas noticed many facts which appear to be referrible to the same pnnci- 

 pies. Memoires prtsente's, vol. ii. 



