Barrett—On Entoptic Vision. 125 
made to change its place, by moving the candle or the perforated 
disc, fresh portions of the retina come into action; and the pre- 
viously fatigued portions rapidly recover their sensitiveness by a 
momentary rest. Hence by continually changing the position 
of the shadow on the retina the figure remains continuously in 
view. 
This experiment shows how quickly the retina becomes 
fatigued, and how quickly it recovers its sensitiveness for the 
perception of small differences of stimulation. This, as we have 
seen, is particularly noticeable in the perception of Entoptic pheno- 
mena. A similar result, as is well known, is seen in viewing the 
image of an extremely faint star; or in detecting the movement 
of a flame in rapid vibration, such as the so-called singing flame. 
In the latter case the separate oscillations of the flame, though they 
may amount to several hundreds per second, can readily be 
detected by a rapid change in the position of the retinal image, 
brought about by moving either the eye or the flame, or by 
rapidly moving a small telescope through which the flame is 
viewed. 
Purkinje’s experiments also reveal the fact that visual sensa- 
tions originate in a part of the retina more deeply seated than 
the vessels, that is, somewhere between those vessels and the choroid 
coat. 
There are other ways of seeing Purkinje’s figures than by a 
moving candle. It is not generally known that long prior to 
Purkinje Dr. Thos. Young described a method by which he was 
able to see the retinal vessels of his own eye. The experiment 
is described in the first paper published by Young, entitled 
“ Observations on Vision,” read before the Royal Society when 
he was only nineteen years old, and published in the Transactions 
of the Society for 1793. Young found that by intermittent and 
forcible pressure on the eyeball he observed “an appearance of 
luminous lines, branched and somewhat connected with each other, 
darting from every part of the field of view towards a centre a 
little exterior and superior to the axis of the eye. This centre 
corresponds to the insertion of the optic nerve; and the appear- 
ance is probably occasioned by that motion of the retina which 
is produced by the sudden return of the circulating fluid into the 
velns accompanying the ramifications of the arteria centralis, 
