present. The arrangements-mechanical and opticalof these instruments I need hardly trouble you with in detail. I have here a variety of spectroscopes kindly lent to me by the maker, Mr. Browning, one with one, one with two, one with three, and one with four prisms. The more prisms we employ, of course the greater dispersion

we get, the more is the light drawn out into its special varieties; and of course the greater also is the intensity of the light which it is necessary to employ in order to get the light to pass through this greater number of prisms.

I will show you first a drawing of ment used by Kirchhoff (Fig. 23).

the actual arrangeThere you see the

prisms employed, four in number, placed one behind. another on a horizontal table of cast iron.

passes through the slit at the end of this tube.

The light

Here (top

of Fig. 23) is an enlarged representation of the slit; on

this is placed a small reflector to enable us to get two spectra-the two superposed. The light passes through the fine vertical slit, the rays are rendered parallel by the lens fixed at the end of the tube (A); it then passes through these four prisms, and the rays thus split up into constituent parts fall on to the telescope (B), at the end of which the eye is placed. This, then, gives you the simplest form of spectroscope, and at the same time the most delicate and complete.

We have here representations as truly painted as possible (see Frontispiece) of what is seen when we allow a light from such coloured flames as those which have been burning to fall on to the retina through a spectroscope properly arranged.

At the top of the diagram (No. 1) is a drawing showing a solar spectrum, and underneath we have the spectra of the alkalies and alkaline earths, potassium (No. 2), sodium (No. 7), and lithium (No. 8), calcium (No. 9), strontium (No. 10), and barium (No. 11), together with the two new metals rubidium and cæsium (Nos. 3 and 4), discovered by Bunsen, about which I shall speak in my next lecture; also the spectra of thallium and indium (Nos. 5 and 6), two other new metals, one of which was lately discovered by our countryman Mr. Crookes. You will perceive in the first place that each of these spectra is different from the rest, although they all possess the common characteristic of containing bright lines or bands, which occur in various portions of the spectrum, and indicate the peculiar kind of light which these various bodies, when brought into a state of glowing gas, emit. The sodium flame when observed by means of the spectroscope exhibits only one bright yellow line; in other words, this light is

monochromatic: sodium vapour gives off light of one degree of refrangibility only, and the spectrum is confined to one very narrow yellow band. The red light, which we saw was due to the presence of lithium, when seen through a prism gives this beautiful red line, together with this paler oranee line. I need not describe the more complicated spectra of strontium, calcium, and barium suffice it to say that they each yield peculiar bright bands, perfectly characteristic of the metal in question, as is seen at once by reference to the drawings.1

For the purpose of enabling any observer unacquainted with the spectra to identify with certainty the presence of any of the foregoing metals by means of their bright lines, and to lay down their positions in his own instrument, the following method of mapping the spectra has been devised by Bunsen. The millimetre scales (Fig. 24) represent the illuminated divisions seen with the scale of the spectroscope (g, Fig. 21): the exact position of the bright lines in any spectrum is shown by the black marks below the divisions; whilst their breadth, intensity, and gradation are indicated by the breadth, depth, and contour of these blackened surfaces. When the spectrum contains a continuous portion of light, this is shown by a continuous black band above the divisions. The positions of the fixed solar lines are given on the first horizontal scale, and those of the most prominent bands in several of the elements are placed as fiducial points at the bottom of the map.2

Now we may ask ourselves, "What improvement is this method of analysis upon our ordinary chemical 1 For the special description of these spectra see Appendix A. p. 68. 2 For further information see Appendix C. p. 88.

methods? What benefit is it to us that barium gives us these peculiar bands, that strontium yields certain different bands, that calcium produces others again? We know already that the chemical reactions of these bodies are very different, and we can detect these substances by ordinary chemical analysis." The answer to this is that the new method is far more delicate than anything which we have hitherto employed, so delicate indeed as almost to pass belief, so that we have hereby obtained a means of examining the composition of terrestrial matter with a degree of exactitude hitherto unknown.

I will try to give you some idea of the delicacy of these spectrum reactions. I can show that the reaction. for sodium is so sensitive that we can detect the presence of sodium in everything. There is not a speck of dust or a mote in the sunbeam which does not contain chloride of sodium. Sodium is a prevailing element in the atmosphere; we are constantly breathing in portions of this elementary substance together with the air which we inhale. Two-thirds of the earth's surface is covered with salt water, and the fine spray which is continually being carried up into the air evaporates, leaving the minute specks of salt which we see dancing in the sunbeam. If I clap my hands, or if I shake my coat, or if I knock this dusty book, I think you will observe that this flame becomes yellow. This is not because it is the hand or coat of a chemist, but simply because the dust which everybody carries about with him is mixed with sodium compounds. If I place in the colourless flame this piece of platinum wire, which has been lying on the table for a few minutes since I heated it red hot, you see there is sodium in it; there,