prominently to those points of a subject where their difficulties were likely to occur. If, indeed, these lectures were delivered nearly in the form in which they are printed, they must have been generally unintelligible even to well-prepared persons, notwithstanding all the assistance which models, drawings, and diagrams could afford.

We have heard it remarked, that no writer, on any branch of science which these lectures treat of, can safely neglect to consult them, so rich is the mine of knowledge which they contain; and it is a well known fact, that many important propositions and discoveries have been more or less clearly indicated in them, which have only been recognised or pointed out when other philosophers discovered them independently, or announced them as their own.

One very striking example of such an anticipation Dr. Young has himself noticed in a review of Dr. Wells's well-known Essay on Dew. In a passage of his fifty-sixth Lecture, he says

"There are frequently some local causes of heat and cold, which are independent of the sun's immediate action. Thus, it has been observed, that when the weather has been clear, and a cloud passes over the plane of observation, the thermometer frequently rises a degree or two almost instantaneously. This has been partly explained by considering the cloud as a vesture, preventing the escape of the heat which is always radiating from the earth, and reflecting it back to the surface."

This observation, combined with others which connect the theory which it indicates with the discoveries of Leslie and Prevost respecting the radiation of heat and the deposition of dew, sufficiently show that Young was, in the year 1807, when this work was published, See Works, vol. ii. p. 424.

R

in full possession of the principle which Dr. Wells, seven years afterwards, so happily brought to bear upon a vast mass of meteorological phenomena; but though Dr. Wells had read and studied, as he himself allows, those parts of the works of Dr. Young which relate to the subject on which he was writing, there is every reason to believe that the passage which we have quoted, and the principle which it involved, had escaped his notice.

"There are, however," says Young, in the Review referred to, "some modern philosophers, who, whether from their own fault, or from that of their hearers and readers, or from both, appear to be perpetually in the predicament of the celebrated prophetess of antiquity, who always told truth, but was seldom understood and never believed. And the author of the lectures in question has not unfrequently reminded us of the fruitless vaticinations of the ill fated Cassandra."

In the year 1802 he was appointed Foreign Secretary of the Royal Society, an office which he held for the remainder of his life, and for which he was well qualified by his knowledge of the principal European languages.

Ten years afterwards, Sir Joseph Banks, in a letter, in which he urged Young to accept the office of Secretary, which was at that time vacant, expressed in very grateful and flattering terms, the sense of the great services he had rendered to the Society, not merely by his contributions to its Transactions, but in the discharge of other duties. The offer was declined from an apprehension lest the tenure of an office conspicuously connected with the cultivation of science would operate prejudicially to him in the practice of his profession.

CHAPTER VI.

OPTICAL DISCOVERIES.-FIRST EPOCH.

THE Contributions made by Dr. Young to the science of Physical Optics, and the relations which they bear to the labours, in the same field, both of his precursors and contemporaries, have been discussed by Dr. Whewell, in his History of the Inductive Sciences, with so just and philosophical an appreciation of their full value and significance, that we should do little more than obscure the lucid picture which he has drawn, if we should attempt to reproduce it in our own colours and with our own less perfect execution. In addition to the sources of information which are open to all, he had access to the interesting series of letters of Young, Arago, and Fresnel, which have been printed in the first volume of the present edition of Dr. Young's works, and which supply some very important facts in the history of the Undulatory Theory at the most critical and, in many respects, the most triumphant period of its progress; and we have found, after a very careful examination of his references to them as well as other documents, that there is nothing to correct and very little to add. In entering now, therefore, upon a short examination of the first and, hereafter, of the second of the two great epochs of Young's optical researches, we shall rarely notice the

labours of other writers, unless they are essentially connected with his own.

What are our ordinary conceptions of a vibration and undulation? We usually connect the first with a reciprocating motion of the particles of the same body, without their transfer in space; with the second, a single vibration, of the same class, in the particles of a medium which is transmitted to similar portions of it in perpetual succession--the same series being renewed as long as the exciting cause of them continues to act.

But how various are the circumstances in which such movements originate, and how complicated are the results which follow from them! How difficult, also, is the accurate conception of a wave and of the mode of its propagation, even when the form which it assumes, the law of its formation, and the properties of the medium which transmits it, are cognisable by our senses! But how much is that difficulty increased, when its almost infinitesimal magnitude and the enormous rapidity of its movement altogether remove it from the sphere of immediate observation!

The pulses of air, which produce the key-note c of the natural scale of music, form an undulation whose breadth is about 212 inches, and of which 64 are propagated in a second of time. We can discover their origin; we can analyse their form and progress, and we are familiar with the medium through which they are conveyed. But if we assume light to consist in the undulations of an ethereal medium, and adopt the consequences to which such a theory leads, we shall find the length of a wave of green light, near the middle of the prismatic spectrum, to be one tenmillionth part of that of the wave producing such a

sound; and that it is propagated with a velocity nine million million times as great, or at the rate of nearly 190,000 miles in a second of time. The human mind is equally embarrassed in dealing with numbers or magnitudes both so great or so small, and it is therefore only through the aid of symbolical language that the analogies which exist between the motions of sound and of light can be brought within the compass of our understanding, and that we are fully enabled to seize upon, not only the points in which they agree, but also those in which they differ.

The objection which Newton most strongly urged against the undulatory theory of light was founded upon the tendency of all undulations, in whatever medium they are excited, to diverge into the surrounding space. Such is confessedly the case with the waves of water, an incompressible fluid, where the force which propagates them is that of gravity. Such he asserts to be the case with waves of sound which are transmitted through a medium of moderate elasticity. The same reasoning and the same analogy would appear to extend, therefore, to the undulations of light, requiring a medium to transmit them of an elasticity proportionate to the velocity which it communicates.

"Are not all hypotheses erroneous," says he, "in which light is supposed to consist of a pression or motion, propagated through a fluid medium? If it consisted in pression or motion, propagated either in an instant or in time, it would bend into the shadow. For pression or motion cannot be propagated in a fluid in right lines beyond an obstacle which stops part of the motion, but will bend and spread every way into the quiescent medium which lies beyond the obstacle. The waves on the surface of stagnating water, passing by the sides of a broad

Principia, prop. xlii., sect. viii., lib. ii. Dr. Young's Works, vol. i. p. 152.