112 Dr. Draper's Further considerations 



wires, between which the spark from a Leyden phial was caused 

 to pass, I placed a lens of quartz, the focus of which for par- 

 allel rays was six inches, and then intercepted the resulting 

 beam by a diaphragm with a circular aperture in it, one- third 

 of an inch in diameter. I had caused an equiangular prism 

 of quartz to be cut and polished from a large and perfectly 

 faultless rock crystal ; it was cut transverse to the axis. This 

 prism I placed in such a way that in dispersing the beam that 

 came through the circular aperture I got rid of double re- 

 fraction 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, oflPered an uniform phos- 

 phorescent surface, which might be set in a vertical plane. 

 When the spark passed I saw that the plate was phosphorescing 

 on those portions where the more refrangible rays had fallen. 



But the transient light of a Leyden spark did not last long 

 enough, nor was the phosphorescence it produced powei'ful 

 enough to enable me to conduct the experiment in a way en- 

 tirely satisfactory. I resorted, therefore, to the brilliant light 

 which is obtained 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 descril/ed. 



As soon as the light was emitted, I marked on the sulphu- 

 ret of lime the beginning 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 phospho- 

 rescing. 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 action which 

 M. Becquerel has so well illustrated in the case of the solar 

 emanations; rays which, however, were first observed in the 

 last century. 



It is necessary to remark, that the rays from the voltaic 

 discharge resemble those from an electric spark in their in- 

 ability to traverse glass. On this observation all the value of 

 the foregoing experiment depends. 



But it can nevertheless be easily proved, that although glass 

 is impervious to the phosphorogenic emanation coming from 



