RETENTION. 
■RETENTION. Whatever be the effect 
produced in the mental organs by the im- 
pressions on the organs of sense, that effect 
can be renewed, though in general with 
diminished vigour, without a repetition of 
the sensible impressions. In other words, 
sensible changes produce a tendency to 
similar changes, which can be repeated 
without the repetition of the external im- 
pressions, and may then be called ideal 
changes. Less generally sensations leave 
relicts behind them, which can be perceived 
without the agency of the external organs 
of sensation, and which are called ideas. 
The power or capacity of the mind by 
which tendencies to ideal changes are re- 
tained, may be called the retentive power. 
That tendencies to a repetition of senso- 
rial changes are thus formed, that ideas are 
thus retained, may be referred to the ope- 
ration of the associative power, and in the 
human being they certainly depend upon 
the same organic causes, whatever those be. 
But in many animals it is decidedly proba- 
ble that sensations leave no relicts behind 
them ; and in man there are, equally pro- 
bably, numerous impressions from external 
objects, which leave no relicts behind them. 
Again, these relicts of sensations can re- 
appear without the agency of external ob- 
jects. Hence it appears preferable to con- 
sider the receiving of sensations, and the 
retaining of ideas, as two separate, though 
intimately connected operations, and as 
implying two separate powers or capacities 
of the mind. This is not done by Hartley, 
who appears to refer both to sensation; 
but it has subjected him to some apparently 
just, though in reality unfounded animad- 
versions of the great northern philosopher, 
Hugald Stewart. Speaking of the pheno- 
mena of memory as not to be intirely ex- 
plained by the law of association, he says, 
{p. 412.) “ The association of ideas con- 
nects our various thoughts with each other 
so as to present them to the mind in a cer- 
tain order, but it presupposes a faculty of 
retaining the knowledge we acquire.” This 
Hartley knew, and has accordingly a dis- 
tinct section on the generation of ideas. 
■Without the retentive power it is obvi- 
ous that man would be a being of mere 
sensation, little, if any, superior, to the 
lowest orders of the animal creation, and 
inferior to many of them. The retentive 
power provides materials for the agency of 
the associative power. Without the reten- 
tive power the associative power would 
wver be called into exercise, and without 
the associative power, the relicts of sensa- 
tion, the effects of the retentive power 
would be of no utility. The operations of 
the retentive power can scarcely be sepa- 
rated from those of the associative power, 
which together constitute the compound 
faculty called memory, for an account 
of which see Philosophy, mental, § 105. 
We have said that the receiving of sen- 
sations, and the retaining the relicts of 
them, seem to depend upon the same or- 
ganic causes whatever they be. In some 
instances sensible changes perceptibly con- 
tinue after the sensible objects are removed. 
Two or three facts, which every one must 
have noticed, or may notice, will illustrate 
this principle. If a piece of stick be burnt 
at one end, and the lighted end be turned 
quickly round in a circle, the luminous 
point will appear to the eye as a complete 
luminous circle ; the changes of the optic 
organs continuing till the image of tlie lu- 
minous point returns to any given point of 
the retina. Again, the sensible changes 
produced by sound, perceptibly continue 
after the external cause ceases. If a sound- 
ing body be struck very rapidly with a stick, 
we do not perceive any interval, and as 
Hartley observes, the most simple sounds 
which we hear, being reflected from the 
neighbouring bodies, consist of a number 
of sounds succeeding each other at different 
distances of time, according to the distances 
of the reflecting bodies. The sensible 
changes produced by the other senses, also 
continue some time after the impressions 
which have been made upon them. If a 
hard body be pressed upon the palm of the 
hand, it is not easy to distinguish, for a 
few seconds, whether it remains or is re- 
moved. And tastes continue to be per- 
ceived long after the sapid material is with- 
drawn. 
This play of the organs, (which however 
is rather to be referred to the external than 
to the mental organs), gives rise, in the case 
of vision, to a number of very singular and 
interesting phenomena, by some pliiloso- 
phers called ocular spectra. A considerable 
variety of them are stated'' by Dr. R. Dar- 
win, of Shrewsbury, at the end of the se- 
cond part of Darwin’s Zoonomia. We 
shall select a few of the most striking. 
Place about half an inch square of white 
paper on a black hat, and looking steadily 
on the centre of it for a minute, remove 
your eyes to a sheet of white paper ; after 
a second or two a dark square will be seen 
on the white paper, which will be seen for 
