x 
330. Dr. Reuben on moving Blood-corpuscles within the Retina 
elevated portion of the membrane corresponding with the foramen 
centrale and limbus luteus, or “yellow spot” 5 
which is the seat of most distinct vision. It is difficult, however, 
sible spot. 
5. Let it be remembered, further, that though the posterior 
layers of the retina are of gray or vesicular nerve matter, yet 
the layer is so thin that the color must be quite ineffective; so 
that the retina through its whole depth is nearly as transparent 
as water. consequence is, that this membrane consisting of 
we may say, a real, though in one direction quite limited, solid- 
. ity of form. 
6. Now the red blood-discs, the most numerous variety, mov- 
ing in the vessels lying directly in front of, and partly in, the 
impressible portion of the retina, and, which, in the small capil- 
laries are probably separated in most instances so as to advance 
they lie with the concave sides transverse to the rays of light, to 
overcome part of the convergency given by the crystalline lens, 
or to cause the minute pencils transmitted by them to diverge. 
If, however, they present themselves edgewise to the light, 7.e., 
if their concavities\are transverse to the paths in which they are 
advancing, then we may suppose, either, that in the direction 
vertical to their path they act as convex lenses; or that, while 
this is the case, certain rays entering at their edges in such part 
as to strike afterward, from within, either of the surfaces that are 
outwardly concave, and of course inwardly convex, would un- 
dergo total reflection, and so an additional effect of concentrating 
or massing together the rays would occur, the whole being 
