O. N. Rood on action of Electric Light on Iodized Plates. 207 
November 20th, 1862, Mr. Chacornac could no longer find the 
least trace of this nebula, while the small star upon which it was 
projected did not present any variation of brightness; and the | 
nebula has since been invisible with the instruments of the Im- 
perial Observatory of Paris. 
We see by the preceding Notices what a degree of activity 
and interest the researches upon nebule now inspire, and also 
how many difficulties they present. It will not probably be 
very long before we may hope to obtain a solution of some of 
the important questions to which they have given rise. 
Art. XVIII.—On the action of very weak Electric Light on the 
Todized Plate; by OGDEN N. Roop, Professor of Physics in 
Columbia College. 
time of taking the picture. - Careful examination of the plate 
of invisible electric brushes, resting on these points, and under- 
some experiments to determine whether weak electric light 
could be photographed. Geisler tubes were used in a dark room, 
and with the aid of Gunther, he succeeded in obtaining good 
photographs of the stratified discharge,' as Prof. Wm. B. Rogers 
ad done some months previously. 
This led me to attempt the study of the electric brush by the 
aid of photography, but as its light is incomparably weaker than 
Toxyline, in which the cotton fibre was somewhat disintegrated, 
and-by its use I finally obtained good photographs of the post- 
tive, as well as of the negative h. An ordinary came 
Was employed, and the exposure | en minutes. The 
minute photographs were then enlarged as. usual, and prints 
made from the enlarged negatives. 
* Pogg. Annalen, vol. exiii, No. vii * This Journal, xxx, 8817. 
