Page:Elementary Text-book of Physics (Anthony, 1897).djvu/452

438 reading and the reading when the grating is in such a position that the reflected image of the slit is seen in the telescope is the angle $$\beta.$$ If the grating be now turned until the light of which the wave length is required is observed, the angle through which it is turned from its last position is the angle $$\theta.$$ If the width $$s$$ of an element of the grating be known, these measurements substituted in the equation give the value of $$\lambda.$$

Wave lengths are generally given in terms of a unit called a tenth metre; that is, 1 metre X 10–10. The wave lengths of the visible spectrum lie between 7500 and 3900 tenth metres. Langley has found in the lunar radiations wave lengths as long as 170,000 tenth metres, and Kowland has obtained photographs of the solar spectrum in which are lines representing wave lengths of about 3000 tenth metres.

Instead of the arrangement which has been described, Rowland has devised a grating ruled on a concave surface, and is thus enabled to dispense with the collimating lens and the telescope.