METHOD.
The marked superiority of these gratings lies in the following features, which have been taken advantage of by Rowland in a masterly manner. Let a circle be de scribed with the radius of curvature of the concave mirror as its diameter. The image of a source s (Fig. 775) of violet light on the circumference of this circle, pro duced by diffraction (not by the ordinary rule for finding the image due to a concave mirror), will be formed at a point z,. on the circumference of the same circle. A corre sponding result is obtained for each wave length, consequently, the first diffracted spectrum lies along the circumference from to Rowland places the eyepiece or camera at o, i.e. z and o must coincide at the centre of curvature of the concave mirror. This arrangement makes z—the part of the spectrum at point o—point s and point x lie at the corners of a right angled triangle, with z s always at right angles to s x. 0 x must be of constant length. This property enables the camera and grating to be fixed at the end of a rigid support. There are now two ways of bringing the various spectra or parts of the spectra to o : (i) by moving the slit along the circumference of the circle indicated, when the spectra will sweep past o in the usual order ; (ii) by keeping the slit fixed and moving the arm o x so that its ex tremities o and x travel along two direc tions at right angles, the slit being at the corner of the right angle. The first
method is manifestly too inconvenient to adopt. The second is obtained in the manner shown by Fig. 776. Placed along the directions s o and s x are two rails on which run the wheels under the platforms supporting the camera and grating. The diagram shows the camera in position for photographing part of the blue spectrum. To get all the lines in focus for any section, the plates must be bent to fit on the cir cumference of the circle mentioned before. As the length of o x in Rowland's appara tus is 43 ft., and the plates used for the solar spectrum are 19 in. long and only in. thick, this is easily done. The sun's rays are thrown by a condensing lens on to a totally reflecting prism and then enter the slit. The gratings used have from 10,000 to 20,000 lines per inch. The adjust ment of the slit has to be very accurate, and its width is not more than in. The spectrum is so long that it has to be photo graphed in several sections.