INDEX OF REFRACTION OF THE PHOSPHOROGENIC RAYS. 213 



it was cut transverse to the axis. This prism I placed in such a way, that in disper- 

 sing the beam that came through the circular aperture I got rid of double refraction 

 and obtained only one spectrum ; this was received on a metal plate, which, having been 

 washed over with gum water, and sulphuret of lime dusted on it, offered a uniform 

 phosphorescent surface, which might be set in a vertical plane. When the spark pass- 

 ed, I saw that the plate was phosphorescing on those portions where the more refrangible 

 rays had fallen. 



980. But the transient light of a Leyden spark did not last long enough, nor was the 

 phosphorescence it produced powerful enough to enable me to conduct the experiment 

 in a way entirely satisfactory. I resorted, therefore, to the brilliant light which is ob- 

 tained when a piece of metal, or, what is far better, the hard variety of carbon which 

 is obtained from the interior of gas retorts, is lowered upon mercury entirely filling a 

 very small open porcelain cup, and the continuous discharge of a voltaic battery passed. 

 The battery used contained fifty pairs of Grove's cells, but a smaller number would 

 probably have been amply sufficient. All the remainder of the arrangement was as just 

 described. 



981. As soon as the light was emitted, I marked on the sulphuret of lime the begin- 

 ning of the red, the centre of the yellow, and the termination of the visible violet ray. 

 Then, stopping the current, I examined on what parts the plate was phosphorescing. 

 The commencement of the glow was between the indigo and the blue; towards the 

 blue it extended far beyond the visible boundaries of the spectrum ; I could not see any 

 divisions or points of maxima on it. The surface of the plate shone all over, except 

 in the region of the less refrangible rays, and there were the traces of the negative ac- 

 tion which M. BECQUEREL has so well illustrated in the case of the solar emanations; 

 rays which, however, were first observed in the last century. 



982. It is necessary to remark, that the rays from the voltaic discharge resemble 

 those from an electric spark in their inability to traverse glass. On this observation all 

 the value of the foregoing experiment depends. 



983. But it can nevertheless be easily proved, that although glass is impervious to the 

 phosphorogenic emanation coming from the voltaic deflagration of any metallic bodies, 

 the observation applies to transient discharges only. A voltaic light, which lasts but 

 a moment, fails to cause phosphorescence through glass in the same way that an elec- 

 tric spark does; but if the discharge is continued, the surface presently begins to glow; 

 and if maintained for several minutes, it shines as brightly as though a piece of quartz 

 had been used. 



984. The inability of an electric spark to cause phosphorescence is connected with 

 its transient duration. The voltaic light enables us at pleasure to imitate the effects 

 of an electric spark, or those of the sun. 



985. The phosphorogenic rays, whether they originate in an electric spark or from 

 the sun, occupying thus the same place in the spectrum, and even exhibiting the same 

 peculiarities as the tithonic rays on the iodide of silver, we have next to determine 

 whether this is an apparent or a positive identity. 



986. Professor HENRY, of Princeton, read a paper before the American Philosoph- 



