Dr. Reuben on Moving Blood-corpuscles within the Retina. 329 
blood-corpuscles, acting as lenses, upon the minute pencils of converg- 
ark 
ing and already almost focalized light entering the retina. 
IT. In the communications already referred to, certain other 
facts and principles were stated, which I will briefly recapitulate, 
= the liberty to present more clearly some points before 
implied: 
ied : 
1. Some of the lines did not remain lucid throughout; but 
before disappearing, along full half their length gave place toa 
distinct black line. This I have repeatedly observed; the change 
is marked and unmistakable. But owing to the brevity of the 
whole movement, I at first erred in judging that it was the last- 
formed half of the line that became black, and that the black 
rtion was the narrower. I have since ascertained that it is the 
Jirst-formed half of the line that becomes black, and either v 
soon, or just before disappearance; and what is strange, this 1s 
uniformly the fact, except that in rare instances, to appearance 
at least, a line is traced that is black throughout and from the 
first. The supposed narrowness of the black line also seems to 
* See a note on the Color of single blood globules, in this No., Scientific Intelli- 
gence— Prysios, 
