738 
A€TION OP THE RETINA AND SENSORIUM. 
co-operation of the sensorium in the act of vision. To these belong not 
merely the act of sensation itself, and the perception of the changes 
produced in the retina, as light and colours, but also the conversion of 
the mere images depicted in the retina into ideas of an extended field 
of vision,—of proximity and distance,—of the solidity (in the geome¬ 
trical sense) and size of objects. To this class of phenomena belong 
also the effects of the reciprocal action of different parts of the sensitive 
apparatus on each other, and many phenomena in the retina either not 
excited by light at all, or not by its immediate action. These different 
phenomena might be treated of under the following heads: 1. Of the 
action of the retina generally, and of the co-operation of the sensorium 
in vision. 2. Of ocular spectra. 3. Of the reciprocal influence of dif¬ 
ferent parts of the retina upon each other. 4. Of the simultaneous 
action of the two eyes. 5. Of the subjective phenomena of vision. 
1. Of the action of the retina generally considered, and of the co-operation 
of the sensorium in vision. 
Action of the retina and sensorium.—It has been shown already, in the Preliminary 
considerations on the Physiology of the Senses, that the retina is not a mere conduc¬ 
tor of external impressions, but itself reacts against these impressions. Light and 
colour are actions of the retina, and of its nervous prolongations to the brain. The 
kind of colour and luminous image perceived depends on the kind of external impres¬ 
sion. With this property of the retina, by virtue of which it becomes when irritated 
the seat of the sensations of colour and light we are so well acquainted, that we found 
upon it all inquiries concerning vision. The vibrations of a fluid existing in all space, 
the ether, when of a certain rapidity, produce in the retina the sensation of a certain 
colour; when of a different degree of rapidity, that of another colour; these colours 
or sensations being modes of reaction of the retina. The simultaneous impression of 
undulations of different rapidity upon the same points of the retina excites the sensa¬ 
tion of white light. These same sensations of colours and light may, however, be 
produced, without the agency of the vibrations of an ether, by mere irritation of the 
retina by means of electricity or mechanical pressure. 
If it be the change produced in the retina which we perceive in vision, we may with 
equal correctness say that in the act of vision the retina feels itself in a particular 
state, or that the sensorium perceives the retina in a particular condition. The condi¬ 
tion of repose of the retina is the cause of the appearance of darkness before the 
eyes; the active state of the retina is the cause of the sensation of the illuminated 
field of vision. Under certain circumstances we see our own retina, and separate 
parts of it, when no images are produced in it by external objects. Besides the spec¬ 
tra produced by pressure and electricity, we have an instance of this in the following 
interesting phenomenon first observed by Purkinje:—If, in a room otherwise dark, a 
lighted candle be moved to and fro, or in a circle, at the distance of six inches before 
the eyes, we perceive, after a short time, a dark arborescent figure ramifying over the 
whole field of vision; this appearance is produced by the vasa centralia distributed 
over the retina, or by the parts of the retina covered by those vessels. There are, 
properly speaking, two arborescent figures, the trunks of which are not coincident, but 
on the contrary arise in the right and left divisions of the field, and immediately take 
opposite directions. One trunk belongs to each eye, but their branches intersect each 
other in the common field of vision. The explanation of this phenomenon is as 
follows:—By the movement of the candle to and fro, the light is made to act on the 
whole extent of the retina, and all parts of the membrane which are not immediately 
covered by the vasa centralia are feebly illuminated; those parts, on the contrary, 
which are covered by those vessels cannot be acted on by the light, and are perceived, 
therefore, as dark arborescent figures. In most persons this experiment succeeds 
readily; but in some individuals the phenomenon is produced with difficulty, or notât 
all. The figures of the vessels appear to lie before the eyes, and to be suspended in 
the field of vision. We have here a distinct demonstration of the axiom, that in vision