This and other reasons compel me to hold that the answer to the question put is, that what has been taken for granted is, in all probability, not true. But before I proceed to give the reasons for the faith that is in me I must, at the risk of being both technical and tedious when I should wish to be neither, lead up to the understanding of them.
The spectroscope, however simple or complex it may be, is an instrument which allows us to observe the image of the slit through which the light enters it, in the most perfect manner. If the light contains rays of every wave-length, then the images formed by each will be so close together that the spectrum will be continuous, that is, without break. If the light contains only certain wave-lengths, then we shall get certain, and not all, of the possible images of the slit, and the spectrum will be discontinuous.
Again, if we have an extremely complex light-source, let us say a solid and a mixture of gases giving us light, and we allow the light to enter, so to speak, indiscriminately into the spectroscope, then in each part of the spectrum we shall get a summation—a complex record—of the light of the same wave-length proceeding from all the different light-waves. But if by means of a lens we form an image of the light source, so that each particular part shall be impressed in its proper place on the slit-plate, then in the spectrum the different kinds of light will be sorted out.
There is a simple experiment which shows clearly the different results obtained. If we observe the light of a candle with the spectroscope in the ordinary manner, that is, by placing the candle in front of the slit at some little distance from it, we see a band of color—a continuous spectrum—and in one particular part of the band we see a yellow line, and occasionally in the green and in the blue parts of the band other lines are observable. Now, if we throw an image of the candle on to the slit—the slit being horizontal and the image of the candle vertical—we then get three perfectly distinct spectra. We find that the interior of the candle, that is the blue part (best observed at the bottom of the candle), gives us one spectrum, the white part gives us another, while on the outside of the candle, so faint as to be almost invisible to the eye, there is a region which gives us a perfectly distinct spectrum with a line in the yellow. In this way there is no difficulty whatever in determining the coexistence of three light-sources, each with its proper spectrum, in the light of a common candle.
We see in a moment that much the same condition of affairs will be brought about if, instead of using a candle, we use an electric arc, in which the pure vapor of the substance which is being rendered incandescent fills the whole interval between the poles, the number of particles and degree of incandescence being smaller at the sides of the arc. We can throw an image of such an horizontal arc on a vertical slit; the slit will give then the spectrum of a section of the arc at right angles to its length. The vapor which exists farthest from the core of the arc has a
