* 
334 Proceedings of the British Association. 
Mackenzie informs us that few symptoms prove so alarming to 
persons of a nervous habit or constitution as muse@ volitantes, 
and that they immediately suppose that they are about to lose their 
sight by cataract or amaurosis. ‘The details which I have sub- 
mitted to you prove that the musce volitantes have no connexion 
with either of those diseases, and are altogether harmless. This 
valuable result has been deduced from a recondite property of di- 
vergent light, which has only been developed in our own day, 
and which seems to have no bearing whatever of an utilitarian 
eharacter ; and this is but one of numerous proofs which the 
progress of knowledge is daily accumulating, that the most ab- 
stract and apparently transcendental truths in physical’science, 
will sooner or later, add their tribute to supply human wants, and 
alleviate human sufferings. Nor has science performed one of 
the least important of her functions, when she enables us either 
in our own case or in that of others, to dispel those anxieties and 
fears which are the necessary offspring of ignorance and error.” 
_ Sir D. Brewster read a notice ‘on the line of visible direction 
along the axis of vision.’ In D’Alembert’s memoir ‘on different 
questions in Optics,’ published in his Opuscules Mathematiques, 
tome i, he has maintained the singular opinion that distant ob- 
jects, like the fixed stars, when viewed directly with both cyes; 
are not seen in their true direction, that is, neither in the direc- 
tion of the rays which they send to the eye, nor of the line (co- 
incident with it) drawn from the point of incidence on the retina 
through the centre of visible direction. The author pointed out 
the fallacy in D’Alembert’s reasoning, and thus established in p- 
position to the opinion of that distinguished philosopher, the law 
of visible direction which he had explained at the Newcastle 
meeting. or 
Dr. Reade exhibited an experiment with an instrument which 
he called an Iriscope, A piece of black polished glass WS rub- 
bed over in part with.a solution of Castile soap; a8 soon @ it 
was dry, the soap was polished off with a glove, until, as far as 
appearances were concerned, the one part of the glass was 4S 
clean as the other. He then blew his breath on the plate through 
a tube about half an inch in bore, and instantly the most vivid 
rings of cdlors (resembling Nobili’s) were exhibited where the 
breath condensed on the part of the glass which had been pre- 
viously soaped: while, on the other part, the condensed breath 
exhibited simply the usual dead’ gray color. 
