REPOUT ON PHYSICAL OPTICS. 349 



But when a dark object is behind the lenses, and the incident 

 light somewhat oblique, the rings immediately change their 

 character, and resemble those of the ordinary reflected system j 

 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 particlef . 

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

 serve 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 

 extent, 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 & 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 rela- 

 tions are unknown. Thus the cycles and epicycles of the 

 Ptolemaic system represented with fidelity the more obvious 

 movements of the planetary 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 glaringly does this blemish show 

 itself in that part of the theory which has been last under con- 

 sideration, that one of its advocates says, " Revera illae vices 

 reflexionis et transitus, cum omnibus additamentis fictitiis, 

 mirabiliores adhuc sunt quam phaenomenon ipsum, ad cujus 

 explicationem in usum sunt vocatse|." The same attribute 

 appears in the broader divisions of the science ; and the several 

 classes of phenomena do not flow from the theory as from one 

 common source, — ^but each has its separate and independent 

 head, and its separate and independent data. In the wave- 



* " Account of some Casesof the Production of Colours," Phil. Trans. 1802. 

 The Ahbe Mazeas noticed many facts which appear to be referable to the same 

 principles, — Memoires presentes, vol. ii. 



t Introduction to Medical Literature, p. 556. 



X Mayer on Newton's Rings, — Gottingen Memoirs, vol. v. p. 22. 



