372 CAROLINE IV. BALDWIN: [Vol. III. 



any curvature, the film holder was curved to conform to the focus 

 of the concave grating. A fence work of horizontal and vertical 

 bars placed back of the film kept it from curving longitudinally, 

 and yet permitted the spectrum to be seen from the back, so that 

 a cleared film could be used for focussing. 



The light used was obtained from a Thomson and Houston arc 

 lamp, which was hung so that the arc could be kept nearly in 

 the horizontal plane in which lay the center of the grating and the 

 central points of the slit and camera box. The carbons were 

 I. cm. in diameter, unplated, and were made with a soft core 

 .3 cm. in diameter. To produce the arc spectra of the metals, the 

 cores were removed and the space was filled with the metallic salt 

 well pounded in. 



In order to prevent fogging of the photographic plates, the 

 lamp L, Fig. i, was placed in the outer of two dark rooms, and 

 the light was reflected into the inner room, which contained the 

 grating, by means of a large concave mirror {M). This mirror 

 was mounted so as to be adjustable about both a horizontal and a 

 vertical axis. Its radius of curvature was twelve feet. The arc 

 light was placed nearly at the center of curvature of the mirror, 

 which was so adjusted that a real, slightly enlarged image was 

 formed on the slit (5). This arrangement was also of advantage, 

 inasmuch as it did away with the use of a converging lens. The 

 arrangement of the apparatus is given in Fig. i, to which refer- 

 ence has already been made. 



The opening between the two rooms was closed on the outer 

 side by a slide provided with a circular opening, and on the inner 

 side by a solid slide, by means of which the light could be entirely 

 shut out. 



A screen near the lamp cut off the direct rays of the opening 

 between the rooms ; and another screen was placed a short dis- 

 tance in front of the slit, to intercept the scattered light. 



Outline of the Work. 



The image of the arc, as examined by the eye, shows sheaths 

 of different colors. The arc is slightly cone-shaped, the apex 



