Dr. Reuben on moving Blood-corpuscles within the Retina. 331 
brought to a focus more quickly or brighter in itself; and thus, 
the dise would project this bright pencil just beyond it into 
the impressible layer, and necessarily carry it along with its 
own motion. But if the capillary, as is sometimes the case, be 
too small, so that the red corpuscle becomes wedged, then it 
has been observed, under other circumstances, to be variously . 
changed in form, usually inclining to globular; and the latter is 
the form also of the few white corpuscles contained in the blood ; 
so that in these instances the effect of a minute spherical lens is 
secured or approximated. 
7. But, are the corpuscles more dense than the serum and the 
nervous mass, so that they can act as lenses? Itis well known, 
first, that the specific gravity of healthy blood slightly exceeds 
that of brain ; and I shall have to assume that the density of the 
retinal substance is not essentially different, as its composition 
and structure are not, from that of brain. But again, in blood 
whose coagulation is delayed for a few minutes after withdrawal 
from the veins, the red corpuscles are found rapidly sinking 
through the liquid, so that the clot thereafter forming is at top 
yellowish only, or of buff color, and the serum is quitered. The 
white corpuscles are to a greater extent entangled in the clot. 
Hence, it must be inferred that at least the red corpuscles have 
a density distinctly greater than that of the blood, and in all 
probability, than that of the retinal mass; and this is all that is 
necessary to show that they must act as lenses upon any light 
which they can transmit. 
8. And this brings me to the question, can the red blood dises 
transmit light of any hues, even green or blue? It must be re- 
collected that, in the minute capillary vessels, these dises can no 
onger advance in masses of several in thickness, but must ad- 
vance singly, or at most in layers of two or three deep. In such 
a case, the blood-dises can hardly present a hue deep enough to 
render them effectively colored, that is, opaque to their comple- 
mentary color. The thickness of two red corpuscles advancing 
side by side with their longer diameters presented in the direc- 
tion of entering rays, would not exceed in all about the ;,;th 
of an inch. Now, let us suppose a solution of the same absolute 
Intensity of red as that of the blood, introduced into a wedge- 
shaped glass of very gradual taper, and in which the film could, 
near the edge, be brought to a thinness of T7ssth of an inch: 
how much of the spectrum would the red medium at this degree 
of tenuity cut off? how much of the intensity of a green or blue 
beam would it destroy? Inappreciably little, it appears to me; 
and this view is sustained by an experiment named in Prof. Rood’s 
per. He says: ‘“ Yellow solutions, when combined with the 
blue glass or blue solutions, render the circulation invisible; and 
it does not re-appear till the yellow solution has been made so 
dilute as barely to preserve a yellow tint, and to transmit the spectrum 
