through an ordinary spectroscope. Thus the light from the tube Z is brought to a focus on the slit t1. It is then made parallel by means of the lens x2 and passes through the prism W, which is filled with bisulphide of carbon. The
FIG. 75 lens x3 forms the spectral images of the slit t1 in the plane of the slit t2. The arm ZW of the spectroscope can be moved so as to bring either the red, the green, or the blue spectral image upon this slit, from which it passes into the instrument.
Fig. 74 is a view of the plan of part of the instrument. The arrangement of surfaces shown diagramatically in Fig. 72 is readily recognized. All of the plates, I may state, instead of being rectangular, have a circular border, because in this form they can be worked true more readily.
Fig. 75 represents a vertical cross-section of the same instrument. It will be noted that the reference plane is divided into sections. This is done in order to enable us to determine very accurately the position of the interference fringes. The two intermediate standards will be recognized at the right.
