Photographing Ultra-Violet Spectra. 101 



ment are seen. The body consists of two parts supported on a 

 wooden stand. The portion M (Plates 12, 13) is a rigid shallow box, 

 in the front of which is the lens in its fitting N. The portion of the 

 body L, a continuation of M, is triangular in form and moveable in 

 two directions. It is made to approach or recede from the lens by 

 the screw K, and to make a greater or lesser angle with the direc- 

 tion of the mean ray X - ^ by turning on the vertical pivot, the 

 centre of which is situated at x. The two portions of the camera 

 are united by a bellows-body of leather of about three inches in 

 length which admits of sufficient play for all the purposes of 

 focussing. The sensitised surface of the glass plate when in 

 position is indicated by the dotted line a-fi. The screws N, N, 

 serve to clamp the " swing-back " of the camera in position. 



It will be seen in the elevation that the camera-body is raised 

 on supports above the iiat stand, but that the camera-back, 

 into which fits the dark-slide, is continuous above and below the 

 camera-body. This arrangement enables one to obtain several 

 photographs on one plate by moving down the frame carrying 

 the dark slide by means of a rack and pinion. Thus in the elevation 

 of the camera-back which is open C— c (Plate 14) is the moveable 

 frame work, D — c^i is the dark slide, B B are the thumb screws, 

 and b the pinion which works in the rack on the front edge of 

 the dark slide. The interior of the dark slide with wires for 

 holding the corners of the sensitive plate are shown by G, g ; 

 while H, h, is the slit in the camera-back through which the 

 spectrum is projected. E and F represent the door at back and 

 the sliding front of dark slide respectively. In the elevation an 

 opening I,i (Plate 12), in the body of the camera, and door to the 

 same J through which is seen the slit H, h, by which the spectrum 

 l)asses to the sensitised plate. In the act of focussing, a glass plate 

 covered with some fluorescent substance, is placed in the dark 

 slide and the lines of the spectrum are viewed by reflection from 

 its surface by looking through the opening I, i. 



This is the most convenient way of viewing the ultra-violet 

 rays, as the coloured spectrum and the obscure rays are visible at 

 the same time. 



The arrangement of prism and collimator tube, the former when 

 imcovered is shown in the elevation of the same. P (Plate 

 15) is the prism, S the aperture for the ordinary rays, T 



