58 Prof. J. Trowbridge on Sioectra of 



Both of these writers appear to have considered the curva- 

 ture ot' the section of the lens in any direction to be the 

 important element in determining tlie position of the focal 

 lines instead of the principle of symmetry. 



I have to thank Prof. Everett for suggesting to me the 

 reason why my equatio'is represent the false focal lines as 

 well as the real; also Mr. Lyndon Bolton for tracing the 

 curve represented by equation (2), and showing that in a 

 particular case it has parabolic asymptotes. 



Yll. Spectra of Gases and Metals at Hif/Ji Temperatures. 

 By John Teowbridge^. 



[Plate IV.] 



ri^HE spectra of metals in atmospheric air are the visible 

 JL evidence of extremely complicated chemical reactions 

 due to the metallic vapour and the gases of the atmosphere. 

 The spectra of gases also in narrow containing vessels of" 

 glass or of quartz are modified by the walls of these vessels 

 when the temperatures of the gases are very high ; moreover, 

 the ordinary method of obtaining photographic spectra either 

 of metals in air or rarefied gases, by long continued discharges 

 produced by the Ruhmkorf coil or transformers, masks certain 

 fundamental reactions. 



It is therefore desirable to study the effect of known 

 quantities of energy successively applied to produce spectra 

 either of metals or gases. This can best be accomplished by 

 charging a condenser to a known amount by a known electro- 

 motive force, and by discharging the condenser between 

 terminals of metals either in air or in gases. If the spectra 

 produced in this manner, by discharges varying from one to 

 any desired number, are photographed on the same plate and 

 treated alike in the same developer, the ground may be pre- 

 pared for some generalization of the extremely complicated 

 reactions I have mentioned. I believe that this method is a 

 fundamental one to use if order is to be brought out of the 

 chaos of spark-spectra. 



I have applied this method in the following manner : — A 

 storage-battery of from ten thousand to twenty thousand cells 

 is employed to charge a condenser '1 to '3 microfarad. By 

 a simple mechanical appliance the condenser is detached from 

 the poles of the battery, and is discharged between suitable 

 terminals. Although it is impossible to avoid a slight spark 



* Communicated by the Author. 



