334 Proceedings of the British Association. 



Mackenzie informs us that few symptoms prove so alarming to 

 persons of a nervous habit or constitution as muscce 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 musccB 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 

 character ; 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 Mathem,atiques, 

 tome i, he has maintained the singular opinion that distant ob- 

 jects, like the fixed stars, when viewed directly with both eyes, 

 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 opr 

 position to the opinion of that distinguished philosopher, the law 

 of visible direction which he had explained at the Newcastle 

 meeting. 



Dr. Reade exhibited an experiment with an instrument which 

 he called an Iriscope. A piece of black polished glass was rub- 

 bed over in part with a solution of Castile soap; as soon as 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 as 

 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 colors (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. 



