The positive rays show, when the pressure is not very low, the line spectrum of the gas through which they pass.
This effect is perhaps best witnessed on viewing a line spectrum such as that of the mercury arc, focused upon a ground glass.
| line spectrum An image of colored lines or bands of light formed in optical spectroscopy, each line representing one of the frequencies in the spectrum of a light source. The light source is usually broken into individual bands by a prism or a diffraction grating. |
