338 Dr. Reuben on Moving Blood-corpuscles within the Retina. 
that the impression of the retina through many of these bodies 
must be decidedly different from that which it receives or yields 
around their place. We may say, generally, that the visible 
corpuscles or cells become so from the fact that in certain ways 
and degrees they complement the effect already impressed on the 
light by the crystalline lens and the humors of the eye. This 
view suggests a ready explanation of the circumstance that media 
of different colors require a convergency of the optic axes toward 
points at different degrees distant from the eyes; since we must 
presume that this is one instance of “associated movements,” and 
one in which, accordingly, the mere telescopic adjustment of the 
place and convexity of the crystaliine lens, will be made to cor- 
respond, involuntarily and accurately, as in ordinary vision, with 
the degree of convergency voluntarily or otherwise given to the 
optic axes, 
The change by which certain of the lucid lines are replaced 
through a part of their length by: black, it is difficult to explain. 
The effect can hardly be that of the accidental or subjective 
production of black (total absence of impression), succeeding a 
previous impression of brightness; for, were it such, it should 
be more general, and follow through the whole length of the 
bright line. Assuming each line to be not less than twenty 
tints. An the caution must be added, that experiments of this 
kind can scarcely be protracted very long at any one time, or re- 
peated continually for weeks or months, with no long intervals 
af rest, without the risk of permanent injury to the delicate 
organization of the eye. 
New York, Jan. 16th, 1861, 
Norz :—WMarch 15.—Since the above was written, Sir D. Brewster's 
paper enumerating “Certain Affections of the Retina” (Philos, Maga- 
zine, Jan. 1861), has appeared, but containing no allusion to vision of the 
blood-di To the list, this, and certain related phenomena quite as 
singular, it appears, may now be added. By 
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