The overlapping of orders in a Fabry-Perot interferometer can be avoided by using two interferometers in series. The fundamental equation shows that the dispersion is independent of the plate separation, while the distance between orders is inversely proportional to it. Thus an instrument with a small separation may be used as a preliminary filter to eliminate some of the orders in one of larger separation. This will not affect the fine structure pattern, and the resolution will be even greater than that due to the larger separation. Such an instrument has been built and satisfies the predictions of the theory. A plate, taken with the green line of mercury, is shown to illustrate the effect of the preliminary interferometer.

• Received 15 November 1926

DOI:https://doi.org/10.1103/PhysRev.29.478