Light from the superpressure mercury lamp with a condenser lens and iris passes through a slit, which is imaged on a distant screen by a 20 cm focal length convex lens. A grating is placed in the light after the slit and the focusing lens. For this one we have been using a triple grating with 2400, 7500, and 15,000 lines per inch. Several orders are visible.
The spectrum is a continuous spectrum with some brighter lines superimposed on the continuum. The photograph shows the first order spectrum in some detail. The blue line at the right is actually ultraviolet, but is rendered visible due to fluorescence of a whitener in the white paper used as a photography backdrop. The photographs of the spectra seen above is overexposed so that more of the background and weaker lines can be seen. This leads to distortion of the colors of the lines, introducing some red into the blue lines and causing them to appear magenta in color. Your eye sees these colors correctly, but cannot see as far into the ends of the spectrum.