8 REPORT——1840. 
of the sun. All coloured objects had the intensity of their colours 
proportionably augmented, and I was thus led to believe that the in- 
crease of colour produced by the partial or total inversion of the head 
arose from the increased quantity of blood thrown into the vessels of the 
eye-ball,— the increased pressure thus produced upon the retina, caused 
from the increased sensibility thus given to the vertical membrane. 
Subsequent observations have confirmed this opinion; and though I 
cannot pretend to have demonstrated it, I have no hesitation in ex- 
pressing it as my conviction that the apparent increase of tint to which 
I have referred is not an optical but a physiological phenomenon. If 
this is the case, we are furnished with a principle which may enable us 
not only to appreciate faint tints which cannot otherwise be recognized ; 
but to perceive small objects, which, with our best telescopes, might be 
otherwise invisible.” 
————- —_———- 
On the Phenomena and Cause of Musce Volitantes. By Sir Davip 
BREWSTER. 
The following are the principal results described in detail in this 
communication.—1. That in persons of all ages, and with the most 
perfect eye, transparent filaments or tubes exist in the vitreous 
humour, and at different distances from the retina. 2. That these 
filaments float in the vitreous humour, moving about with the mo- 
tion of the head. 3. That these filaments are seen by means of 
their shadows on the retina, and are most distinctly visible in diver- 
gent light, their shadows being bounded by fringes produced by dif- 
fraction or inflexion. 4. That the real musce, resembling flies, are 
knots tied, as it were, on these filaments, and arising from sudden 
jerks or motions of the head, which cause the long floating filaments 
to overtop and run into knots. 5. By making experiments with the 
head in all positions, and determining the limits of the motions of the 
musce, by measuring their apparent magnitude, and producing double 
images of them by means of two centres of divergent light, the author 
was able to determine their exact place in the vitreous humour, and to 
ascertain the important fact, that the vitreous humour in the living 
human eye is contained in cells of limited magnitude, which prevent 
any bodies which they contain from passing into any of the adjacent 
cells. Sir David Brewster concluded with the following observations. 
“JT have dwelt thus Jong on the subject of Musce volitantes, not 
only because it is an entirely new one, but also on account of its prac- 
tical utility. Myr. Mackenzie informs us that few symptoms prove 
so alarming to persons of a nervous habit or constitution as Musce 
volitantes, and that they immediately suppose that they are about to 
lose their sight by cataract or amaurosis. The details which I have 
submitted to you prove that the Musce volitantes have no connexion 
with either of these diseases, and are altogether harmless. This valu- 
able result has been deduced from a recondite property of divergent 
light, which has only been developed in our own day, and which might 
