Gases and Metals at Iligli Temperatures. 



59, 



at tlie moment of makinof contact with the receiving system, 

 an approximately equal quantity of electricity is communi- 

 cated to this system at each discharge. The method also 

 permits of a definite control. 



In order to photogra})h on the same plate the spectra pro- 

 duced hy successive discharges, the photographic plate must 

 be slid vertically from one position to another at the focus of 

 a Rowland grating. The most convenient arrangement for 

 the study ot" gaseous spectra is to employ a grating of short 

 focus, and to inclose it in a light-tight box for compactness, 

 and to dispense with a dark room. I have employed the 

 method of mounting the grating in such a manner that the 

 normal to the ruled surface passes through the slit. The 

 camera swings on the arc of a circle described by an arm of 

 half the radius of the grating. This arm is pivoted at a 

 point halfway between the ruled surface and the slit. 



In fig. 1, G is the grating, M the point midway between 



Fio-. 1. 



a 



m 



SXit 



the grating and the slit, C the camera swinging on the arc 

 described from M. Fig. 2 gives a side view and elevation of 

 the camera ; P is the photographic plate, an opening- 

 closed by a slide operated by a lever-arm which engages 

 with A. The plate-holder, closed by another slide S, can 

 move up or down in parallel ways. In fig. 1 is shown a 

 lever-arm LM with a fulcrum at F. This lever lifts another 

 lever AM which lifts or closes the shutter A, fig. 2. The 



