A low-pressure sodium lamp is used to project the line spectrum of sodium. This one is not real bright, but can be seen if the screen isn't too far away. The advantage is that because it operates at a lower temperature it shows that the yellow light from a sodium lamp has a single yellow component.
The light is defined by a slit, which is imaged on a distant screen after the light passes through the diffraction grating. Gratings of 2000 and 7500 lines per inch are furnished.