On the Sums of Neutral and Periodic Series. 437 



p;lass; and differ from those of an electric spark, or galvanic 

 discharge, in which that peculiarity is deficient. '-J 



I have also remarked some curious cases of spectral api^ 

 pearances, — they are analogous to those instances to which I 

 first drew public attention in 1840, and which, at a later dat^^ 

 M. Moser brought before the British Association. They are 

 interesting, as affording an ocular proof of " secondary radia- 

 tion." The following experiments may serve as illustrations: — 



Place a key, or any other opake object, before a sensitive 

 phosphorescent surface, and having made that surface glow 

 intensely, by a galvanic discharge, between charcoal points, 

 continued for two or three minutes, on removing the key an 

 image of it will of course be seen. This image in a short time 

 will disappear. Then shut the plate in a dark place, where 

 no light can have access to it in the daytime. If in a day or 

 two the surface be carefully inspected, in the dark, no trace 

 of anything v/ill be visible upon it ; but if it be laid on a piece 

 of warm iron, a spectral image of the key is suddenly evolved. 



It is still more curious, that a number of these latent images 

 may co-exist on the same surface. Provide a phosphorescent 

 surface, on which the latent image of a key, impressed a day 

 or two before by the galvanic discharge, is known to exist. 

 Take some other object, as a metal ring, and setting it before 

 the surface, discharge at a short distance a Leyden jar. The 

 phosphorus shines all over, save on those portions shaded by 

 the ring; it exhibits therefore an image of" that body. This 

 image soon fades away, and totally disappears. Set the plate 

 now upon a piece of warm iron ; it soon begins to glow, and 

 the image of the ring is first reproduced ; and as it declines 

 away, the spectral form of the key gradually unfolds itself, 

 and after a time it totally vanishes. ^ 



A series of spectral images may thus exist together on a 

 phosphorescent surface, and after remaining there latent for 

 a length of time, they will come forth in their proper ordfir 

 on raising the temperature of the surface. ' '^i^' 



Tlife idea that phosphorescence is merely the light of eleo^^ 

 trie discharge from particle to particle seems to me wholly JiJi' 

 compatible with such results. idR 



LXX. On the Evaluation of the Sums of Neutral and Periodic' 

 Series. By J. R. Young, Professor of Mathematics in 

 Belfast College. ^ ^;:;;;;^^ ' ibidw'ay^Sbd^rfq 



[Continued from p. 366.] * * , ^j^-^f 



IVf ETHODS for the evaluation of what have improperly 

 ^^^ been called neutral series — those series namely which 



