I)R W. OKOPFREY DlTFFIELD ON THE 



The arc was more easily maintained between 50 and 200 atmospheres pressure than 

 between 1 and 50 atmospheres. Of all the photographs taken, that at 200 atmospheres 

 gave least trouble, the arc burning steadily for several seconds with dazzling 

 brightness. Extension to higher pressures would be quite possible if the difficulties 

 attendant upon the fracture of the glass and quartz windows could be overcome. As 

 has already been described* in connection with experiments made with the copper 

 arc, the straining of the windows resulted in splinters of glass breaking off and ruining 

 the surfaces, and sometimes in the complete breakdown of the window. The present 

 pressure cylinder cannot be safely used at higher pressures, but a similar but stronger 

 one should present no great difficulties. The running of the arc itself is a simple 

 matter. 



4. The Photographs: (1) Method of Exposure. As in the previous work with 

 the iron and copper arcs, the comparison spectrum under atmospheric pressure was 

 photographed in the central strip of a plate (20 inches long by 2j inches broad) with 

 the spectrum under pressure above and below it. To ensure that no accidental 

 displacements were produced the comparison spectrum was photographed before and 

 after the one under pressure. The arc was operated by the writer and the mirrors 

 by an assistant. 



The following photographs have been obtained : 



Plates : Imperial Flashlight. Developer Imperial Pyro-Metol Standard. Exposure 

 minutes at 5 atmospheres to 5 minutes at 200 atmospheres; but this 

 ;tle ind lC ation of the relative intensities because the width of the slit was different 

 in nearly every exposure. 



(2) Description of the P/o^.-Pkte 1 illustrates the behaviour of the silver arc 

 Ferent pressures. The plate includes the region X = 4020 to X = 4320 

 .tographs which are full-size positive reproductions of the originals 

 n order of increasing pressure from the top at 1 atmosphere to the 



) atmospheres, the central strip of. each being at normal atmospheric 

 pressure. 1 



, A, 



