axis is exactly underneath the slit of the second wet cardboard. By turning the arm, the path of the pencil is exactly marked out, and one can verify that it is quite unique, and is accompanied by no lateral pencil, such as diffraction could eventually produce in the case of large wave-lengths.
A grating is then placed in front of the slit of the second wet cardboard (for instance, a Brunner grating of 200 lines per mm.). If, now, the emerging pencil is explored by turning the arm which bears the phosphorescent sulphide, the existence of a system of diffraction fringes is confirmed, just as with light, only these fringes are much closer together, and are sensibly equidistant. This already indicates that "N" rays have much shorter wave-lengths than luminous radiations.
The angular distance of the fringes, or what amounts to the same thing, the rotation of the arm corresponding to the passage of the phosphorescent slit from one luminous fringe to the next, is very small. It is therefore determined by the method of reflection, with the aid of a divided scale and telescope, a
