60 Meetings of the Scientific Association of Great Britaan. 
ascertain by experiment, and r=100. The communication, made 
by Mr. Potter at Cambridge, had for its object particularly to show 
the coincidence between the results calculated by the formula, and 
those furnished by experiments with the glass of antimony. 
In discussing the importance of these observations, we were ac- 
cidentally led to speak of the new photometer invented by the au- 
thor, Mr. Ritchie, whose photometer is well known, made some 
very interesting remarks on the organ of sight,* and upon the mistakes 
to which it is liable. 
Mr. Herschel read a paper on the absorption of light, of which 
he promised me an extract. He alsorepeated with the greatest suc- 
cess, an experiment on the interferences of sonorous rays, mentioned 
in his article on acoustics in the Encyclopedia Metropolitana, and 
which consisted in vibrating two diapasons, perfectly in unison above 
a glass vessel about eight inches in height, and less than two inches 
in diameter: a little water was at the bottom of the glass; the dza- 
pasons successively vibrating above the glass, gave a continuous 
sound, and when they vibrated simultaneously, we heard very rapid 
and very distinct intermissions of sound. 
Mr. Hamilton showed the principal results respecting conical re- 
fraction, to which he has been led by his genious views and his 
elegant analysis; and Professor Lloyd mentioned the result of some 
of his observations which fully confirmed what Mr. Hamilton had 
discovered by his formulas. 
At a special meeting which I attended with Messrs. Herschel, 
Brewster, Powell, Christie, etc., Mr. Wheatstone showed a very in- 
genious experiment; its object was to determine whether the ap- 
pearance of a light is instantaneous, or has an appreciable duration, 
and if so to measure this duration. For example, he endeavored 
to ascertain if an electric spark has an appreciable duration. To 
determine this, Mr. Wheatstone took a circle of paste board, which 
he divided into several sectors alternately white and black ; he then 
caused the circle to revolve in its own plane around a fixed axis, 
and the result was that by this rotation the surface of the circle ap- 
peared grayish, on account of the duration of the impression of light 
* IT recollect when at Mr. Gartner’s, whose establishment at London is so well 
known to geographers, that I found him engaged in showing some persons pres- 
ent, that the hand is endowed with more sensibility than the eye. His demonstra- 
tion consisted in tracing, by his hand and with a simple rule, lines, so near togeth- 
er and so tine, that the eye could not distinguish nor count them but by means ofa 
magnifying glass. 
