This is the best adjustable slit spectroscope we have seen. By varying the amount of light passing into the prism, this portable spectroscope is able to resolve even very close or dim spectral lines. This makes identifying a light source easier and more accurate. Use for examining the spectral composition of white light, sodium vapor or mercury vapor street lamps, LED's, neon signs, and more. Similar to Oliver Sacks' pocket spectroscope which he used as a youth. The metal spectroscope is 20 mm in diameter, 130 mm long when fully extended and has an adjustable light aperture at one end. Light passes through the slit, through a prism and is observed as a spectrum at the other end. Each quality spectroscope comes in its own beautiful wooden box.