Meetings of the Scientific Association of Great Britain. 61 
upon the retina. This determined, if the circle is put in motion in 
a perfectly dark chamber, which is suddenly illuminated by an elec- 
tric spark or by a discharge from a Leyden jar, we shall see, very 
distinctly, the black and white sectors as’if the circle were in a state 
of perfect rest, notwithstanding the rapidity of rotation which may 
be given it. We must then conclude that the circle has been en- 
lightened only during an instant infinitely short. Yet the image on 
the retina is so vivid and continuous as to render the image of the 
circle very distinct. We must suppose that if an electric spark had 
an appreciable duration, we should see the revolving circle in sev- 
eral successive positions, and it would be impossible clearly to dis- 
tinguish its image.* : 
The nature of these investigations, which depend on the duration 
of sight, gave me an opportunity to speak of Mr. Plateau’s research- 
es in Belgium, who, in following out the very curious researches of 
Messrs. Roget and Faraday, has made a very ingenious little in- 
strument which he calls a phantascope, and which has been since 
imitated in France in a very imperfect manner, under the name of 
phenakistiscope, and in London under that of phantasmascope. 
Magnetism.—Magnetism was not neglected, the desire was ex- 
pressed of seeing observations on the inclination and intensity of the 
magnetic needle multiplied, as they are unfortunately still very rare. 
Mr. Christie of Woolwich. showed very clearly the importance of 
corrections, and especially of those with reference to the inequality 
of temperature, a subject with which he has been recently occupied. 
There was also a discussion on the inequality of the magnetic force, 
which Mr. Kuppfer thinks he has observed between the summit and 
base of mountains, contrary to previous observations, and which | 
have had an opportunity of verifying in the Alps, with Mr. Necker 
Saussure, who had the-goodness to take a part with me; and which 
Professor Forbes has since equally confirmed, but of which the re- 
sults have not as yet been published. Researches of this kind were 
especially recommended to the attention of observers. I believe it 
will not be less interesting to verify a remarkable result which 
Mr. Necker has deduced from my observations ; it is that the mag- 
* Prof. Joseph Henry, of Princeton, N.J., repeated the interesting experiment 
mentioned above, on the instantaneous illumination of an object by an electric 
spark. The effect is most easily shown by the common philosophical toy called 
the phenakistoscope. If one of the discs of this instrument be put in rapid motion, 
and then illuminated by a discharge from a Leyden jar, it will appear to be at rest 
with the picture on it distinctly visible. 
