Page:The wonders of optics (1869).djvu/257



now come to speak of an instrument which may fairly rank, after the telescope and microscope, as one of the most wonderful discoveries of modern optical science. By its means we have not only discovered four new elementary bodies, which are found in certain minerals in inconceivably small quantities, but we have also determined the chemical composition of some of the remotest stars and nebulæ.

In 1701 Newton discovered that if an ordinary ray of white light was admitted through a small hole into a dark chamber, and thence passed through a triangular prism, it became decomposed into a coloured band, known as the solar spectrum. As we have already explained that this decomposition is caused by the different coloured rays that make up white light being bent unequally by the action of the prism, we trust the following explanations will be readily understood. In 1802 Dr. Wollaston, an English philosopher, discovered that by using a narrow slit, instead of a round hole, the resulting spectrum was no longer continuous, but was divided at intervals by dark lines extending across it in a direction parallel to the edges of the prism. These lines attracted considerable attention at the time, but it was not until 1815, that Fraunhofer, an optician of Munich, investigated them with accuracy. He mapped