86 Scientific Proceedings, Royal Dublin Society. 
subject in 1860.1. Prof. Rood also detected the movement of 
small bodies which he took for blood-corpuscles, but their size, 
estimated from their projected shadows, was about double that of 
blood-corpuscles. 
It is quite easy to see this phenomenon if a bright sunlit sky 
be looked at through a cobalt-blue glass held close to the eye. 
A rapid succession of bright specks like minute fire-flies are seen 
darting swiftly onward in numerous broken curved paths of short 
radius. I hope shortly to publish a note giving further particulars 
and the actual dimensions of these corpuscles. 
Other American observers, Prof. Rogers and Dr. Reuben, 
in 1861, also drew attention to these streams of particles, and 
attributed them to moving blood-corpuscles. Helmholtz, who 
repeated the experiment, states that he could see the phenomenon 
very well with his right eye, a little to the left of the point of 
fixation. He came to the conclusion that the circulation of the 
blood and groups of corpuscles are really thus entoptically seen ; 
but only when small obstacles in the retinal circulation occur, 
thus a local and temporary stoppage in the circulation takes place. 
An agglomeration of blood-corpuscles and a variable velocity in the 
flow of blood through the smaller capillaries are thus produced, 
which render the phenomenon visible. The fact has hitherto 
been overlooked that Dr. Thomas Young noticed this as long ago 
as 1793. He found that by prolonged pressure on the sclerotic 
and interrupted pressure on the cornea, temporary stoppage of the 
circulation in the retinal vessels is produced; and the sudden 
return of the blood enables the branching capillaries and also the 
flow of the blood to be perceived.® 
(6). It is well known that mechanical and electric stimulation 
of the retina give rise to luminous appearances to which the name 
of phosphenes has been given. Pressure on any small part of the 
sclerotic is transmitted to the retina, and causes a bright phosphene 
to be projected in the opposite direction to the pressure. Young 
describes and explains this phenomenon; and Helmholtz and 
1 See American Journal of Science (Silliman’s Journal), vol. xxx., Sept. 1860 ; also 
a second paper on the subject by Prof. Rood in vol. xxxi. 
* Helmholtz, Optique Physiologique, pp. 221 et seq. 
3 Philosophical Transactions, 1793, p. 160. 
