MR. CHARLES I)E WATTEVILLE ON FLAME SPECTRA. 



143 



(1.) Rowland Grating Spectroscope. 



This spectrograph, constructed for Professor SCHUSTER and made to his design hy 

 the Cambridge Scientific Instrument Company, is enclosed within a light-tight caw, 

 the greatest length of which is rather larger than 1 metre. At one end is the 

 grating, fixed vertically on a platform, which can be moved either in a vertical or in 

 a horizontal direction by means of three clamping screws. For this purpose the 

 platform is drilled with three large holes, through which pass the vertical bolts. The 

 lower ends of these bolts are screwed into a metal plate, which is itself firmly fixed 

 to the base of the box. Two nuts on each bolt serve to clamp the platform at any 

 desired height. At the other end of the case there is a dark slide, in which the 

 photographic film is placed, or in which the eye-piece used in the adjustment of the 

 grating and for eye observations may be fitted. The eye-piece is held in place by 

 springs, and may be moved along two grooved iron rails which have been bent to the 

 requisite curvature. When films are used, they are firmly pressed against these rails 

 by means of a piece of cui-ved wood, which closes the slide securely. 



There are, in addition, two circular openings in the case, corresponding respectively 

 to the positions to be occupied by the slit, when it is desired to have on the film 

 either the first-order spectrum or the second-order spectrum in their full extent. A 

 system of regulating screws, placed near each of these two openings and against 

 which the tuta carrying the slit is made to bear, allow the exact position of the slit 

 to be re-found, lx>th as regards possible vertical 

 displacements and horizontal displacements, 

 when the slit has to be moved from one aperture 

 to the other. 



The grating itself is a fine Rowland concave 

 grating of 15,000 lines to the inch, and having 

 a ruled length of 3 '5 inches. Its radius of 

 curvature is I metre, and on the films used a 

 length of 1 millim. corresponds to a difference 

 of wave-length of about 16 '8 Angstrom units. 



Adjitstment of the Orating. As is well 

 known, the series of images of the slit formed 

 by diffraction will all lie on a cylindrical 

 surface to which the slit and the grating are 

 both tangents. The diameter of this cylinder 

 is equal to the radius of curvature of the 

 grating, and the normal RM (fig. 3) to the grating should coincide with a diameter 

 of the cylinder. To perform the adjustment, one begins by ascertaining that the 

 centre of the ruled jx>rtion of the grating is in a horizontal plane which divides 

 the film into two equal halves ; then one places a wire cross at a point M,, situated 



