such a proceeding may often he 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
will 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 phmnomena 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 efi’ects 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- varies the whole universe. 2. That lmdulations are excited in this ether whenever a body becomes luminous. And, 3. That the sen- sation of difiemnt 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 difi’erent 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 afiecting in any degree its ad-
