overlap, but produce their impressions side by side of each other, and their examination and investigation is therefore rendered possible.
The elements of spectroscopy will be familiar to most of you, but you will forgive me if I briefly allude to some points, which, though well known, are of special importance in the considerations which I wish to bring before you to-night.
When a body is sufficiently hot it becomes luminous, or, to speak in scientific language, the vibrations which are capable of producing a luminous sensation on our retina are increased in intensity as the temperature is raised, until they produce such a sensation. By means of a strong electric current I can in the electric lamp raise a piece of carbon to a high temperature. When looked at with the unaided eye it seems whitehot, but, when I send the rays through a prism and project them, as I do now, on a screen, you see a continuous band of light. This fact we express by saying that the spectrum of the carbon poles in the electric lamp is a continuous one. You see side by side the different colors known to you by the familiar but incorrect name of "the rainbow-colors"; and the experiment teaches you that the carbon pole of the electric lamp sends out rays in which all wave-lengths which produce a luminous sensation are represented.
But, if now I introduce into the electric arc a small piece of a volatile metal, you see no longer a continuous band of light. The band is broken up into different parts. Narrow bands or lines of different colors are separated by a space sometimes black, sometimes slightly luminous. The metal has been converted into vapor by the great heat of the electric current, and the vibrations of its molecules take place in distinct periods, so that the waves emanating from it have certain definite lengths. If the molecule could only send out one particular kind of waves, I should in its spectrum only see one single line. We know of no body which does so, though we know of several in which the possible periods of vibration are comparatively few; the spectrum of these will, therefore, contain a few lines only. Thus we have two different kinds of spectra, continuous spectra and line-spectra. But there is a certain kind intermediate in appearance between these two. The spectra of "fluted bands," as they are called, appear, when seen in spectroscopes of small dispersive powers, as made up of bands, which have a sharp boundary on one side and gradually fade away on the other. When seen with more powerful instruments, each band seems to be made up of a number of lines of nearly equal intensity, which gradually come nearer and nearer together as the sharp edge is approached. This sharp edge is generally only the place where the lines are ruled so closely that we can no longer distinguish the individual components. The edge is sometimes toward the red, sometimes toward the violet, end of the spectrum. Occasionally, however, the fluted bands do not show any sharp edge whatever, but are simply made up of a series of lines which are, roughly speaking, equi-
