Miscellanies. 167 



centrale, a crescent-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 tTiem, 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. 



The foregoing beautiful experiment of exhibiting to its own vision 

 the blood vessels of the eye, was first described, it appears, by Stein- 

 buch, 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. — Mem. 



4. Hardness of Lead. — It results from the experimeifts of M. Co- 

 riolis, that lead fused and cast in the open air is of variable hardness, 

 and 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. — Idem. 



5. On the power of horses ; Jy B. Bevan. — (Phil. Mag.) — "The 

 mean force exerted by one hundred and forty four horses at various 

 ploughing matches at Woburn and Ashbridge, was one hundred and 

 sixty three pounds each horse, and although the speed was not par- 

 ticularly noted, it could not be less than two miles and a half per hour. 



