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such a proceeding may often be the means of eliciting some further 

 information, which progressively may tend to the advancement of 

 science. The immortal Newton has given us a striking example of 

 this in his Theory of Light, which, should the principle he assumed 

 prove ultimately erroneous, his investigation and mode of reasoning 

 ^vill yet remain an everlasting monument of acutehess and ingenuity, 

 which it is likely will ever be found the best source of information to 

 those who shall engage in this delicate branch of natural philosophy. 



Under this impression, Dr. Young, having resolved to contemplate 

 the subject of light and heat, in the present Lecture, proposes to take 

 a general survey of what is extant, using the materials which, chiefly 

 through Newton's means, are now at hand, and at the same time to 

 add some new experiments of material importance in the investi- 

 gation, in hopes thereby to establish a general principle which may 

 apply to all the phenomena hitherto discovered. 



The Newtonian system of emanation, though illustrated in so 

 masterly a manner by its author, partly on account of the stupen- 

 dous velocity it implies, has been ever thought liable to difficulties, 

 which could not be satisfactorily obviated. Accordingly another 

 hypothesis, namely, that of an ethereal fluid, producing its effects 

 either by an undulatory motion or by a continued pressure, had been 

 substituted by some, without however entering in a methodical man- 

 ner into the abstruse disquisition necessary to establish their theories. 

 This arduous disquisition our author engages in, in favour of the un- 

 dulatory system ; and it is no less curious than satisfactory, that, in 

 carefully examining the writings of Newton, there are abundance of 

 passages which prove that he was strongly impressed with ideas 

 which singularly favour this theory. 



In the first part of the Lecture, our author enumerates these pas- 

 sages, and adduces them in support of the three following hypotheses. 

 1 . That a luminiferous ether, rare and elastic in a high degree, per- 

 vades the whole universe. 2. That undulations are excited in this 

 ether whenever a body becomes luminous. And, 3. That the sen- 

 sation of different colours depends on the frequency of vibrations ex- 

 cited by light in the retina. It is here to be observed that, speaking 

 of the motion of this ether, Newton uses the term vibration instead 

 of undulation, which two words manifestly convey different mean- 

 ings, the one being the alternate motion of a pendulum, and the other 

 that of waves which protrude each other. It is likewise obvious, as 

 to the motion of the retina, that it must rather be of the vibratory 

 than of the undulatory nature, the frequency of the vibrations depend- 

 ing on the constitution of the substance limited to the sensation of 

 colours. 



These three hypotheses, which may be called essential, are here 

 shown to be literally parts of the more complicated Newtonian 

 system. But a fourth is now advanced, which appears diametrically 

 opposite to that of Newton, and differs in some measure from any 

 that has been hitherto proposed by other writers, although the author 

 does not consider this difference as affecting in any degree its ad- 



