64 
rOPULAR SCIENCE REVIEW. 
ceived. The man suffering from muscae volitantes explains the 
form of the shadowy things he sees with the utmost exactitude, 
he may (I have known such a case) give to the appearances fanci- 
ful names from their forms ; yet it is not at any time supposed 
that the seeings are realities : the man wlio tells you a red 
object is colourless, or is of different colour to what all other 
men call red, is considered, however persistent he may be in his 
opinion, peculiar only and deluded : the man who explains that 
he sees but the half of an object, or that he sees two objects 
when there is but one before him, is at once accepted as in- 
correct in his observation: the man who, under opium or 
haschish, receives the impression of being in rooms of infinite 
space, of grasping in one sweep of apprehension incalculable 
intervals of history, is held to be for the time of disordered 
mind : and the man who, under the poison of alcohol, turns the 
simplest of objects into the likeness of the fiend, is credited 
with obvious derangement so long as he thus misinterprets what 
exists before him. Yet there are many persons who, recogms- 
ing such everyday truths to the full, accept other hallucinatory 
phenomena, of a similar origin, as actual external realities, and 
who, once believing it, adhere to the opinion they have formed 
more determinately than to any ordinary fact or business with 
which they are hourly concerned. The story is an old one : — 
John Absolute believed lie could not be deceived, 
When to prove his own belief he took the pains ; 
So he vowed he’d seen a ghost, though he’d felt it was a post, 
And his head had paid the forfeit for his brains.” 
The illusions depending upon changes of functions in the 
receiving nervous surface of an organ of sense, or in the con- 
ducting cord, are comparatively simple. It is when we come 
to consider the reception and the fixing of impressions in the 
brain that the profoundest difficulties arise. Here we pass, with 
ease, out of the domain of current physical science into what is 
but useless speculation, unle.«s we are ever on our guard in 
thought. I shall touch, consequently, on but few subjects; on 
such as are nearest to the physical basis of research. 
The brain receives and retains external impressions brought 
to it through the senses. In the exercise of this function it 
may become unduly impressionable, and may be the seat of 
illusion. Under these circumstances, one particular impres- 
sion may so overrule every other impression that it shall 
persistently present itself. Sometimes a sudden impression 
is made upon the brain so potently that it is stamped, as it 
were, in persistent relief, coming forward at any time — but 
specially when the mind is unoccupied or is weakened — with 
all the force of a new reality. The distinguished French 
