Miscellanies. : 167 
centrale, a erescent-formed image is occasionally observed ; its ap- 
pearance depends on the position of the light with respect to the axis 
of the eye; for instance, when the light is placed below the eye, the 
image appears on looking downwards, and in general it appears on 
looking towards the light and disappears on looking from it. 
The following is the probable explanation of this beautiful phe- 
nomenon: Were the blood vessels which are spread on the anterior 
surface of the retina entirely opaque, they would prevent the trans- 
mission of light to the nervous matter beneath them, and their dis- 
tribution would be constantly visible ; but they are transparent, and 
in ordinary cases the intensity of the light which passes through them 
does not materially differ from that which falls directly on the retina. 
When however the retina is fatigued by a strong light, the veins be- 
come visible, because the retina is rendered insusceptible to a portion 
of the light they transmit; but this effect is only momentary, for those 
Parts which are thus shaded from the more intense light promptly re- 
cover their usual susceptibility and the images vanish; but they may 
again be made perceptible by displacing them on the retina; and by 
making them constantly change their places, the images may be ren- 
dered permanent. The momentary appearance of these images may 
be frequently observed on looking at a strong light immediately after 
waking in the morning, and may be reproduced several times by suc- 
cessively shutting and opening the eyes. ia 
The foregoing beautiful experiment.of exhibiting to its own vision 
. vessels of the eye, was first described, it appears, by Stein- 
bueh, in his Physiolie der Sinne, 1811. It has since been brought 
into notice by Dr. J. Purkinje, Professor of Physiology at the Uni- 
versity of Breslau, 1823.—Idem. 
4. Hardness of Lead.—It results from the experiments of M. Co- 
Niolis, that lead fused and cast in the open air is of variable hardness, 
oné that to obtain it with a true and constant power of resistance, it 
Must be cast out of contact of air, and drawn off from the bottom of 
the Mass.— Jem, 
5. On the power of horses ; by B. Bevan.—(Phil. Mag. )—* The 
Mean force exerted by one hundred and forty four horses at various 
“oUughing matches at Woburn and Ashbridge, was one hundred and 
“xy three pounds each horse, and although the speed was not par- 
Warly noted, it could not be less than two miles and a half per hour. 
