Used as a Sodium vapour light source. These are 2mm and 4mm holes, 4mm x 20mm vertical slit. The lamp power supply housing provides two intensities of operation. The lower intensity provides the more pure sodium vapour performance because the gas in the lamp runs cooler and is a more predominantly monochromatic sodium source. Using a spectroscope, the sodium 'doublet' is clearly visible and this lamp is suitable for general purpose monochromatic work in the classroom when used at the low power setting. The high power setting is useful to demonstrate the change in overall colour as the various other wavelengths begin to appear. Operating voltage 220/240V.AC. 50 Hz.