THE ABSORPTION AN'I) larfSSrON OK All;. 



A hollow cone, d, very accurately groviiul to fit uj).)!! />, is capable of turning round 

 It to the extent of its opening, d^ d^, and carries the camera. 



Fig. 7 shows the camera in cross-section. The jilate-holdei-, drawn in the 

 cross-section, must be shoved into the inner cone, />', through the slot, h', in dark- 

 room light. When the cone is turned, the plate-holder comes before the opening, 

 in', and is exposed to the rays from the prism. A certain amount of air Ts 



Fig. 6. 



Fig. 5. CoNMXTioN with Geissler Tuhe. Naturae Size. 



Cross-Section of Prism- Holder. 

 Half Natural Size. 



unavoidably introduced when the plate-holder is shoved in ; and in case this inter- 

 feres, it has to be pumped out before the exposure begins ; and it will sometimes 

 even be requisite to wash out the entii-e interior of the spectrogra])h with 

 hydrogen. 



Working in dark-room light involves many inconveniences; and I have, there- 

 fore, lately provided the plate-holder with a brazen 

 sheath which is shoved into a vei'tical groove made in 

 h'. In this way, the plate-holder can be introduced 

 into the camera, quite light-tight, in full daylight. 



Fig. 8 is the elevation of the spectrograph, t is 

 the prism-stand with the prism, h, upon it. It rests, 

 free to turn, upon a conical pivot of the little table, h, 

 which is rigidly connected by a screw-thread witii the 

 steel axis, g; and this in turn carries the alidade,/, 

 at the bottom outside the vacuum. The apparatus is 

 connected with the air-pump by means of the ground- fig. 7. Cross-Sf.ction of Camera. 



n rni , ji 1 J ] < w^..,„. Half Natural Size. 



glass cock, 0. The two thumb-screws, t^ and ?2, serve 



to focus the lenses of the collimator and telescope respectively. At the right-hand 

 side of them the camera is seen in section, showing the photogi'aphic plate, i', 

 with its plate-holder. The micrometer, .$•', above it (1 division = 0.1 mm.), serves 

 to move the plate in its own plane parallel to the refracting edge of the prism. 

 This is requisite in case several exposures are to be made successively without 

 changing the plate. 



