When a material is heated to incandescence it emits light that is characteristic of the atomic makeup of the material. In the original spectroscope design in the early 19th century, light entered a slit and a collimating lens transformed the light into a thin beam of parallel rays. A prism then refracted the beam into its spectrum. This image is then viewed through a tube with a scale that was transposed upon the spectral image, enabling its direct measurement. Particular light frequencies give rise to sharply defined bands on the scale which can be thought of as fingerprints. The element sodium has a very characteristic double yellow band known as the Sodium D-lines at 588.9950 and 589.5924 nanometers.
With the development of photographic film, the the more accurate spectrograph was created. It was based on the same principle as the spectroscope, but it had a camera in place of the viewing tube. In recent years the electronic circuits built around the photomultiplier tube have replaced the camera, allowing real-time spectrographic analysis with far greater accuracy. Such spectral analysis, or spectroscopy, has become an important scientific tool for analyzing the composition of unknown material.