note belonging to the forks or strings, that note will be heard in full intensity, except in so far as the strings (merely as obstacles) intercept the passage of the sound. Such a note will be heard almost as powerfully on the other side of the room as if there had been no tuningforks or wires present. But as soon as he plays the particular note which belongs to all the forks or all the strings, it comes to be just the question of the pendulums or magnets, or the two tuning-forks which I have just shown you. The contents of the room gradually absorb each a portion of the sound which reaches it, and are set into vibration by it. If there be enough of them they take all the energy of the sound, and of course completely prevent the sound from passing through the medium, except in so far as they give it out themselves.

Here, then, is a medium which of itself can give out one definite note, and one note alone, when it is a source of sound; but which, when it is employed as a sort of sifter of sound, can sift out from a mixed or confused sound only that particular note. That then is mechanically or physically the analogy to which we shall have to reduce the fundamental principles of spectrum analysis.

LECTURE VIII.

RADIATION AND ABSORPTION.

History of the discovery of the Physical Basis of Spectrum Analysis. First result of Spectrum Analysis applied to non-terrestrial bodies;-There is Sodium gas in the Sun's Atmosphere. Elaborate experiments of Stewart and Kirchhoff. Identity of Light and Radiant Heat. Distinctive characters of a particular ray. Application of Carnot's principle to establish the equality of radiating and absorbing powers. Black, transparent, and perfectly reflecting bodies.

I ENDED my last lecture by considering various modes of transference of energy of vibration from one body to another. I took in particular three cases, in the first of which the transference took place through a solid body, in the second the vehicle was ordinary air, and in the third case it was the medium which propagates magnetic and electric actions. But in every one of these cases we found that the condition which is absolutely necessary for a complete handing over of the energy of one vibrating body to another, whatever be the intervening medium of communication, was that the time of vibration of the second body should be adjusted to be exactly equal to the time of vibration of the body which had the energy at first.

I then went on to suppose a finite space to be filled with a number of such vibrating bodies, all tuned (as it were) to vibrate in precisely the same time; and I showed you that if we considered a space so filled to act as a medium, it would be such as when set in

vibration would give one perfectly definite sound, or note of one definite pitch, and that it would be competent on the other hand to absorb precisely that particular note and no other. All other musical sounds would pass through it without marked interruption (unless its transverse dimensions were very great); but if that particular note were played in its neighbourhood, none of its vibrations could pass through the medium. They would, if the medium were only deep enough, be entirely absorbed and retained by the medium.

Now, that is the dynamical principle which led Professor Stokes, about the year 1850, to the first distinct anticipatory statement of the physical basis of spectrum analysis. In order that I may make quite intelligible to you how he made the application to the analogy between the behaviour of certain bodies as regards light, and the behaviour of those tuning-forks and strings as regards sound, it will be necessary for me to make a slight digression. That digression has reference to the different refrangibilities of the different colours or wavelengths of light.

It was one of Newton's simplest and yet greatest discoveries in optics that when a beam of white sunlight passes through a prism it is divided into its various components. The components have different wavelengths, or different times of vibration, or different refrangibilities, for these three properties vary together, -in virtue of which their paths, when they pass out of the glass prism, differ in direction; and therefore they all spread out from each other into the graduated series of colours which Newton called the solar spectrum. Newton's first method of forming the spectrum was complete so far as the object he had in view was con

cerned; but it is not sufficient to enable us to make that searching scrutiny of the composition of sunlight which alone would enable us to tell whether certain perfectly definite coloured rays are wanting in it or not. In order to achieve this it is absolutely necessary to make some optical arrangement by which no two portions of coloured rays of different refrangibilities shall be allowed to overlap one another (as it were) in the spectrum.

Newton's first method was simply to cut a round hole in the window-shutter of a dark room, and allow the beam of sunlight which entered by it to pass through his prism, and then to be received upon a white screen. Make, then, the simplest possible supposition, so as to save complexity at the commencement of our explanation; suppose that there were only two distinct kinds of homogeneous light in sunlight,—that it were made up, for instance, of homogeneous red and green in proper proportions to make white. By this method of experimenting we should have had on the screen, before inserting the prism, a circular spot of white sunlight. After the interposition of the prism this would be decomposed into two circular spots; a red spot depending on the one kind of light, and a green spot depending on the other; both displaced (but the green most) from the original position of the white spot. But, as we know, sunlight consists not merely of a particular kind of red and a particular kind of green, but of almost every shade of colour intermediate between and beyond these limits on each side; and it is therefore evident that the method gives superposition of equal round spots of light of gradually increasing refrangibility, with their centres arranged continuously (or almost

continuously) along a straight line on the screen; so that there must be a constant overlapping of a great many of these successive spots at any one point of the spectrum, and therefore it must be practically impossible by this method to detect the absence of any one particular shade of colour.

Now, though the optical method which Newton1 devised for the purpose of avoiding this difficulty is a very simple one, it deserves a word or two, as it will help you to understand the experimental illustrations I mean to give in my next lecture. Instead of using a round hole we now use a narrow slit whose sides are perfectly parallel to each other, and which can be made (by proper mechanical adjustment) as wide or as narrow as we choose. The light from the sun or electric lamp, or whatever source we employ, comes through this slit as a thin sheet, and falls upon an achromatic lens; that is, a lens which behaves in almost precisely the same way to all the differently coloured rays falling upon it. It is usually convenient to place the lens at such a distance from the slit that at exactly the same distance beyond the lens an image of the slit, equal to it in size, will be formed on the screen. If, then, sunlight pass through the slit, and fall upon the lens, we shall have, on a screen placed at the proper distance, simply an intensely bright white line, consisting of all the different rays belonging to sunlight. But if you interpose in the course of those rays, just after they come through the lens, a prism, with its edge parallel to the slit, the effect will be a change of direction of those cones of rays which are converging towards the image. The prism will most refract the 1 Optics, Book I. Part i. Exp. 11, Illustration.