visibility curve, as it may be termed—we can draw conclusions as to the character of the radiations which cause the interference phenomena, even when such investigation is beyond the power of the best spectroscopes. In order to make the method (it may perhaps be called the method of light-wave analysis) an accurate process, it is necessary, in the first place, to produce a number of visibility curves from known sources. Thus, for example, we may take two lines corresponding to the sodium lines, and produce their visibility curve, as we did before, by adding up the separate fringes and obtaining the resultant; we may then take three or four or any number of lines, and determine the corresponding visibility curves. Each of these, instead of being a single line, may have an appreciable breadth, and the brightness of the line may be distributed in various ways within the breadth.
Now, the process of adding up such a series of simple harmonic curves (for the interference fringes are represented by simple harmonic curves) is very laborious. Hence the instrument shown in Fig. 56, called a harmonic analyzer, was devised to perform this work mechanically. It looks very complex; in reality it is very simple, the apparent complexity arising from the considerable number of elements required. A single element is shown in Fig. 57. A curved lever which is pivoted at o is represented at B. One cud of this lever is attached to the collar of the eccentric A. When this eccentric revolves, it therefore transmits to the lever B a motion which is very nearly simple harmonic. The amount of the motion which is communicated to the writing lever u is regulated by the distance of the connecting rod R from the axis o. When the connecting rod is on one side of the axis the motion is positive; when on the other side the motion would be negative. The end of this lever is connected to another lever x, and the farther
